ARM: sunxi: Add earlyprintk support using R_UART (sun6i/sun8i)

sun6i/sun8i have a UART in the RTC block group, which can be used
as an early console. This is most useful on sun8i as UART0 is muxed
with MMC0, which is not available if we boot from MMC.

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