ARM: sunxi: Add basic A31 support

Add a new sun6i machine that doesn't do much for now.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[wens@csie.org: Enabled MMC configs and adapted Kconfig system]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>