Added driver to control the external power output

This PR adds support to control the external power output from controllers like nice!nano, nRFMicro etc

I have implemented based on my understanding of Pete suggestion on this feature.

Testing done:

    Tested by enabling and disabling the ext_power from application and verified
    Verified the application does not crash with boards that does not have ext_power support
    Note:
    I did not test this in nice!nano since I don't have the boards. Will get help from others once the behavior PR is up

Next Steps:

    Create a behavior PR to control enable/disable ext_power

app/src/ext_power_generic.c

@@ -0,0 +1,91 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_ext_power_generic
+
+#include <device.h>
+#include <init.h>
+#include <drivers/gpio.h>
+#include <drivers/ext_power.h>
+
+#if DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT)
+
+#include <logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+struct ext_power_generic_config {
+    const char *label;
+    const u8_t pin;
+    const u8_t flags;
+};
+
+struct ext_power_generic_data {
+    struct device *gpio;
+    bool status;
+};
+
+static int ext_power_generic_enable(struct device *dev) {
+    struct ext_power_generic_data *data = dev->driver_data;
+    const struct ext_power_generic_config *config = dev->config_info;
+
+    if (gpio_pin_set(data->gpio, config->pin, 1)) {
+        LOG_WRN("Failed to set ext-power control pin");
+        return -EIO;
+    }
+    data->status = true;
+    return 0;
+}
+
+static int ext_power_generic_disable(struct device *dev) {
+    struct ext_power_generic_data *data = dev->driver_data;
+    const struct ext_power_generic_config *config = dev->config_info;
+
+    if (gpio_pin_set(data->gpio, config->pin, 0)) {
+        LOG_WRN("Failed to clear ext-power control pin");
+        return -EIO;
+    }
+    data->status = false;
+    return 0;
+}
+
+static int ext_power_generic_get(struct device *dev) {
+    struct ext_power_generic_data *data = dev->driver_data;
+    return data->status;
+}
+
+static int ext_power_generic_init(struct device *dev) {
+    struct ext_power_generic_data *data = dev->driver_data;
+    const struct ext_power_generic_config *config = dev->config_info;
+
+    data->gpio = device_get_binding(config->label);
+    if (data->gpio == NULL) {
+        LOG_ERR("Failed to get ext-power control device");
+        return -EINVAL;
+    }
+
+    if (gpio_pin_configure(data->gpio, config->pin, config->flags | GPIO_OUTPUT)) {
+        LOG_ERR("Failed to configure ext-power control pin");
+        return -EIO;
+    }
+
+    return 0;
+}
+
+static const struct ext_power_generic_config config = {
+    .label = DT_INST_GPIO_LABEL(0, control_gpios),
+    .pin = DT_INST_GPIO_PIN(0, control_gpios),
+    .flags = DT_INST_GPIO_FLAGS(0, control_gpios)};
+
+static struct ext_power_generic_data data = {.status = false};
+
+static const struct ext_power_api api = {.enable = ext_power_generic_enable,
+                                         .disable = ext_power_generic_disable,
+                                         .get = ext_power_generic_get};
+
+DEVICE_AND_API_INIT(ext_power_generic, DT_INST_LABEL(0), ext_power_generic_init, &data, &config,
+                    APPLICATION, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &api);
+
+#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */

app/src/main.c

@@ -15,16 +15,25 @@ LOG_MODULE_REGISTER(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <zmk/matrix.h>
 #include <zmk/kscan.h>
 #include <zmk/display.h>
+#include <drivers/ext_power.h>
 
 #define ZMK_KSCAN_DEV DT_LABEL(ZMK_MATRIX_NODE_ID)
 
 void main(void) {
+    struct device *ext_power;
     LOG_INF("Welcome to ZMK!\n");
 
     if (zmk_kscan_init(ZMK_KSCAN_DEV) != 0) {
         return;
     }
 
+    // Enable the external VCC output
+    ext_power = device_get_binding("EXT_POWER");
+    if (ext_power != NULL) {
+        const struct ext_power_api *ext_power_api = ext_power->driver_api;
+        ext_power_api->enable(ext_power);
+    }
+
 #ifdef CONFIG_ZMK_DISPLAY
     zmk_display_init();