gpio: ath79: Move to the generic GPIO driver

Drop most of the code in favor of the generic MMIO GPIO driver.
As the driver now depend on CONFIG_GPIO_GENERIC also add a Kconfig
entry to make the driver optional.

We leave the base pointer and lock in the data struct because they are
needed for the IRQ support.

Signed-off-by: Alban Bedel <albeu@free.fr>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 77df866..020a44a 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -126,6 +126,15 @@
 	  driver for GPIO functionality on Promontory IOHub
 	  Require ACPI ASL code to enumerate as a platform device.
 
+config GPIO_ATH79
+	tristate "Atheros AR71XX/AR724X/AR913X GPIO support"
+	default y if ATH79
+	depends on ATH79
+	select GPIO_GENERIC
+	help
+	  Select this option to enable GPIO driver for
+	  Atheros AR71XX/AR724X/AR913X SoC devices.
+
 config GPIO_BCM_KONA
 	bool "Broadcom Kona GPIO"
 	depends on OF_GPIO && (ARCH_BCM_MOBILE || COMPILE_TEST)