sunxi: Enable CMD_GPIO on all sunxi boards

We have CONFIG_CMD_GPIO=y in almost all sunxi boards, but after
its Kconfig conversion it has ended up missing on some recently
added boards.

Simply select it for ARCH_SUNXI, so that we get it on all
sunxi boards for both a consistent user experience and simpler
defconfig files.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>