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/kscan.c

@@ -58,8 +58,7 @@ void zmk_kscan_process_msgq(struct k_work *item) {
     }
 }
 
-int zmk_kscan_init(char *name) {
-    const struct device *dev = device_get_binding(name);
+int zmk_kscan_init(const struct device *dev) {
     if (dev == NULL) {
         LOG_ERR("Failed to get the KSCAN device");
         return -EINVAL;