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/gpio/Kconfig.595

@@ -0,0 +1,24 @@
+# 595 GPIO configuration options
+
+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+DT_COMPAT_ZMK_GPIO_595 := zmk,gpio-595
+
+menuconfig GPIO_595
+    bool "595 Shift Register SPI driver"
+    default $(dt_compat_enabled,$(DT_COMPAT_ZMK_GPIO_595))
+    depends on SPI
+    select HAS_DTS_GPIO
+    help
+      Enable driver for 595 shift register chip using SPI.
+
+if GPIO_595
+
+config GPIO_595_INIT_PRIORITY
+    int "Init priority"
+    default 75
+    help
+      Device driver initialization priority.
+
+endif #GPIO_595