Add custom keymap for darp9

Despite the darp9 supporting an RGB backlit keyboard, the keyboard
option was removed and it ships with a white-only LED backlit keyboard
instead. The keypad `/` still has the cycle symbol printed on it, so
replace the color cycling function with the keyboard brightness
function used by other white-only LED keyboards (the 14" TKL ones).

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/board/system76/darp9/board.mk

@@ -14,6 +14,7 @@ CONFIG_SECURITY=y
 
 # Include keyboard
 KEYBOARD=15in_102
+KEYMAP=darp9
 
 # Set keyboard LED mechanism
 KBLED=rgb_pwm