gru: kevin: define GPIOs used on both platforms

The same GPIOs are used on both platforms, definitions are added an a
new .h to make it easier to re-use them across the code.

BRANCH=none
BUG=chrome-os-partner:51537
TEST=panel backlight still enabled on Gru as before. The rest of the
     GPIOs are used in the upcoming patches.

Change-Id: I54ef3e8dd79670bdb037baeec91430113d11bcc1
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: c58788026f28af52c650da0159b93d97269ca4a9
Original-Change-Id: I1a6c5b5beb82ffcc5fea397e8e9ec2f183f4a7e0
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/346219
Original-Tested-by: Shunqian Zheng <zhengsq@rock-chips.com>
Reviewed-on: https://review.coreboot.org/18176
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/mainboard/google/gru/mainboard.c

@@ -371,7 +371,7 @@ static void enable_backlight_booster(void)
 
 void mainboard_power_on_backlight(void)
 {
-	gpio_output(GPIO(1, C, 1), 1);  /* BL_EN */
+	gpio_output(GPIO_BACKLIGHT, 1);  /* BL_EN */
 
 	if (IS_ENABLED(CONFIG_BOARD_GOOGLE_GRU) && board_id() == 0)
 		enable_backlight_booster();