sunxi: move SUNXI_GPIO to Kconfig

Probably for no particular reason SUNXI_GPIO was still defined the "old
way", in header files only.

Introduce SUNXI_GPIO to the Kconfig file in drivers/gpio to remove
another line from our dreadful config_whitelist.txt.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com> # Pine-H64
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index fa1c997..4532ed3 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -215,6 +215,12 @@
 	  of 'anonymous' GPIOs that do not belong to any device or bank.
 	  Select a suitable value depending on your needs.
 
+config SUNXI_GPIO
+	bool "Allwinner GPIO driver"
+	depends on ARCH_SUNXI
+	help
+	  Support the GPIO device in Allwinner SoCs.
+
 config XILINX_GPIO
 	bool "Xilinx GPIO driver"
 	depends on DM_GPIO