ARM: sunxi: Add support for uart0 on port F (mmc0)

Allwinner SoCs provide uart0 muxed with mmc0, which can then be used
with a micro SD breakout board. On the A23, this is the only way to
use uart0.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>