Documentation: sunxi: Update Allwinner SoC documentation (A31/A31s/A23)

Lately we have received documentation for A31 and A31s, in addition to
A23 documentation which was received earlier but not added.

Add these to the README, and update to reflect that A31 and A23 are
supported.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>