refactor: Move away from deprecated label usages.

* Move away from DT_LABEL.
* Move to DEVICE_DT_GET for non-behavior device access.
* Move various drivers to `gpio_spec_dt` and `DT` related macros.
* Remove mcp23017 while at it, since better upstream driver is
  available.

app/include/zmk/kscan.h

@@ -6,4 +6,6 @@
 
 #pragma once
 
-int zmk_kscan_init(char *name);
+#include <zephyr/device.h>
+
+int zmk_kscan_init(const struct device *dev);