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/src/main.c

@@ -17,12 +17,10 @@ LOG_MODULE_REGISTER(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <zmk/display.h>
 #include <drivers/ext_power.h>
 
-#define ZMK_KSCAN_DEV DT_LABEL(ZMK_MATRIX_NODE_ID)
-
 void main(void) {
     LOG_INF("Welcome to ZMK!\n");
 
-    if (zmk_kscan_init(ZMK_KSCAN_DEV) != 0) {
+    if (zmk_kscan_init(DEVICE_DT_GET(ZMK_MATRIX_NODE_ID)) != 0) {
         return;
     }