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>

drivers/gpio/Makefile

@@ -24,7 +24,7 @@ obj-$(CONFIG_GPIO_ALTERA)  	+= gpio-altera.o
 obj-$(CONFIG_GPIO_AMD8111)	+= gpio-amd8111.o
 obj-$(CONFIG_GPIO_AMDPT)	+= gpio-amdpt.o
 obj-$(CONFIG_GPIO_ARIZONA)	+= gpio-arizona.o
-obj-$(CONFIG_ATH79)		+= gpio-ath79.o
+obj-$(CONFIG_GPIO_ATH79)	+= gpio-ath79.o
 obj-$(CONFIG_GPIO_BCM_KONA)	+= gpio-bcm-kona.o
 obj-$(CONFIG_GPIO_BRCMSTB)	+= gpio-brcmstb.o
 obj-$(CONFIG_GPIO_BT8XX)	+= gpio-bt8xx.o