ARM: sunxi: Add support for using R_UART as console

The A23 only has UART0 muxed with MMC0. Some of the boards we
encountered expose R_UART as a set of pads.

Add support for R_UART so we can have a console while using mmc.

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