refactor: Move drivers into properly module.

* Align our driver module layout to properly match Zephyr conventions,
  allowing proper CMake setup to amend the library for each type of driver.

app/module/drivers/kscan/kscan_gpio.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2022 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <zephyr/device.h>
+#include <zephyr/drivers/gpio.h>
+#include <zephyr/dt-bindings/gpio/gpio.h>
+#include <zephyr/sys/util.h>
+
+struct kscan_gpio {
+    struct gpio_dt_spec spec;
+    /** The index of the GPIO in the devicetree *-gpios array. */
+    size_t index;
+};
+
+/** GPIO_DT_SPEC_GET_BY_IDX(), but for a struct kscan_gpio. */
+#define KSCAN_GPIO_GET_BY_IDX(node_id, prop, idx)                                                  \
+    ((struct kscan_gpio){.spec = GPIO_DT_SPEC_GET_BY_IDX(node_id, prop, idx), .index = idx})
+
+struct kscan_gpio_list {
+    struct kscan_gpio *gpios;
+    size_t len;
+};
+
+/** Define a kscan_gpio_list from a compile-time GPIO array. */
+#define KSCAN_GPIO_LIST(gpio_array)                                                                \
+    ((struct kscan_gpio_list){.gpios = gpio_array, .len = ARRAY_SIZE(gpio_array)})
+
+struct kscan_gpio_port_state {
+    const struct device *port;
+    gpio_port_value_t value;
+};
+
+/**
+ * Sorts a GPIO list by port so it can be used with kscan_gpio_pin_get().
+ */
+void kscan_gpio_list_sort_by_port(struct kscan_gpio_list *list);
+
+/**
+ * Get logical level of an input pin.
+ *
+ * This is equivalent to gpio_pin_get() except that, when iterating through the
+ * pins in a list which is sorted by kscan_gpio_list_sort_by_port(), it only
+ * performs one read per port instead of one read per pin.
+ *
+ * @param gpio The input pin to read.
+ * @param state An object to track state between reads. Must be zero-initialized before the first
+ * use.
+ *
+ * @retval 1 If pin logical value is 1 / active.
+ * @retval 0 If pin logical value is 0 / inactive.
+ * @retval -EIO I/O error when accessing an external GPIO chip.
+ * @retval -EWOULDBLOCK if operation would block.
+ */
+int kscan_gpio_pin_get(const struct kscan_gpio *gpio, struct kscan_gpio_port_state *state);