gpio: ath79: Add support for the interrupt controller

Add support for the interrupt controller using GPIOLIB_IRQCHIP.
Both edges isn't supported by the chip and has to be emulated
by switching the polarity on each interrupt.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 3c1bb91..f52b259 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -131,6 +131,7 @@
 	default y if ATH79
 	depends on ATH79 || COMPILE_TEST
 	select GPIO_GENERIC
+	select GPIOLIB_IRQCHIP
 	help
 	  Select this option to enable GPIO driver for
 	  Atheros AR71XX/AR724X/AR913X SoC devices.