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/Kconfig b/arch/mips/ath79/Kconfig
index 5bc480e..185a8d6 100644
--- a/arch/mips/ath79/Kconfig
+++ b/arch/mips/ath79/Kconfig
@@ -5,6 +5,7 @@
 config ATH79_MACH_PB44
 	bool "Atheros PB44 reference board"
 	select SOC_AR71XX
+	select ATH79_DEV_GPIO_BUTTONS
 	select ATH79_DEV_LEDS_GPIO
 	help
 	  Say 'Y' here if you want your kernel to support the
@@ -21,6 +22,9 @@
 config SOC_AR913X
 	def_bool n
 
+config ATH79_DEV_GPIO_BUTTONS
+	def_bool n
+
 config ATH79_DEV_LEDS_GPIO
 	def_bool n