MIPS: ath79: add common GPIO buttons device

Almost all boards have one or more push buttons connected to GPIO lines.
This patch adds common code to register a platform_device for them.

The patch also adds support for the buttons on the PB44 board.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Cliff Holden <Cliff.Holden@Atheros.com>
Cc: Kathy Giori <Kathy.Giori@Atheros.com>
Patchwork: https://patchwork.linux-mips.org/patch/1954/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/ath79/Makefile b/arch/mips/ath79/Makefile
index f41c029..344e9ab 100644
--- a/arch/mips/ath79/Makefile
+++ b/arch/mips/ath79/Makefile
@@ -16,6 +16,7 @@
 # Devices
 #
 obj-y					+= dev-common.o
+obj-$(CONFIG_ATH79_DEV_GPIO_BUTTONS)	+= dev-gpio-buttons.o
 obj-$(CONFIG_ATH79_DEV_LEDS_GPIO)	+= dev-leds-gpio.o
 
 #