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/arch/arm/Kconfig b/arch/arm/Kconfig
index 8754197..60c4a07 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -936,6 +936,7 @@
 	select SPL_STACK_R if SPL
 	select SPL_SYS_MALLOC_SIMPLE if SPL
 	select SPL_SYS_THUMB_BUILD if !ARM64
+	select SUNXI_GPIO
 	select SYS_NS16550
 	select SYS_THUMB_BUILD if !ARM64
 	select USB if DISTRO_DEFAULTS