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_composite.c

@@ -0,0 +1,112 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define DT_DRV_COMPAT zmk_kscan_composite
+
+#include <zephyr/device.h>
+#include <zephyr/drivers/kscan.h>
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#define MATRIX_NODE_ID DT_DRV_INST(0)
+#define MATRIX_ROWS DT_PROP(MATRIX_NODE_ID, rows)
+#define MATRIX_COLS DT_PROP(MATRIX_NODE_ID, columns)
+
+struct kscan_composite_child_config {
+    const struct device *child;
+    uint8_t row_offset;
+    uint8_t column_offset;
+};
+
+#define CHILD_CONFIG(inst)                                                                         \
+    {.child = DEVICE_DT_GET(DT_PHANDLE(inst, kscan)),                                              \
+     .row_offset = DT_PROP(inst, row_offset),                                                      \
+     .column_offset = DT_PROP(inst, column_offset)},
+
+const struct kscan_composite_child_config kscan_composite_children[] = {
+    DT_FOREACH_CHILD(MATRIX_NODE_ID, CHILD_CONFIG)};
+
+struct kscan_composite_config {};
+
+struct kscan_composite_data {
+    kscan_callback_t callback;
+
+    const struct device *dev;
+};
+
+static int kscan_composite_enable_callback(const struct device *dev) {
+    for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
+        const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
+
+        kscan_enable_callback(cfg->child);
+    }
+    return 0;
+}
+
+static int kscan_composite_disable_callback(const struct device *dev) {
+    for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
+        const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
+
+        kscan_disable_callback(cfg->child);
+    }
+    return 0;
+}
+
+static void kscan_composite_child_callback(const struct device *child_dev, uint32_t row,
+                                           uint32_t column, bool pressed) {
+    // TODO: Ideally we can get this passed into our callback!
+    const struct device *dev = DEVICE_DT_GET(DT_DRV_INST(0));
+    struct kscan_composite_data *data = dev->data;
+
+    for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
+        const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
+
+        if (cfg->child != child_dev) {
+            continue;
+        }
+
+        data->callback(dev, row + cfg->row_offset, column + cfg->column_offset, pressed);
+    }
+}
+
+static int kscan_composite_configure(const struct device *dev, kscan_callback_t callback) {
+    struct kscan_composite_data *data = dev->data;
+
+    if (!callback) {
+        return -EINVAL;
+    }
+
+    for (int i = 0; i < ARRAY_SIZE(kscan_composite_children); i++) {
+        const struct kscan_composite_child_config *cfg = &kscan_composite_children[i];
+
+        kscan_config(cfg->child, &kscan_composite_child_callback);
+    }
+
+    data->callback = callback;
+
+    return 0;
+}
+
+static int kscan_composite_init(const struct device *dev) {
+    struct kscan_composite_data *data = dev->data;
+
+    data->dev = dev;
+
+    return 0;
+}
+
+static const struct kscan_driver_api mock_driver_api = {
+    .config = kscan_composite_configure,
+    .enable_callback = kscan_composite_enable_callback,
+    .disable_callback = kscan_composite_disable_callback,
+};
+
+static const struct kscan_composite_config kscan_composite_config = {};
+
+static struct kscan_composite_data kscan_composite_data;
+
+DEVICE_DT_INST_DEFINE(0, kscan_composite_init, NULL, &kscan_composite_data, &kscan_composite_config,
+                      APPLICATION, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &mock_driver_api);