underglow-layer: use devicetree & clean up code

app/include/dt-bindings/zmk/rgb_colors.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2021 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define GREEN 0x00ff00
+#define RED 0xff0000
+#define BLUE 0x0000ff
+#define TEAL 0x008080
+#define ORANGE 0xffa500
+#define YELLOW 0xffff00
+#define GOLD 0xffd700
+#define PURPLE 0x800080
+#define PINK 0xffc0cb
+#define WHITE 0xffffff
+#define ______ 0x000000