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>