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/sensor/battery/battery_common.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr/drivers/sensor.h>
+#include <stdint.h>
+
+struct battery_value {
+    uint16_t adc_raw;
+    uint16_t millivolts;
+    uint8_t state_of_charge;
+};
+
+int battery_channel_get(const struct battery_value *value, enum sensor_channel chan,
+                        struct sensor_value *val_out);
+
+uint8_t lithium_ion_mv_to_pct(int16_t bat_mv);