ARM: dts: sun8i: Add pinmux set for uart0

uart0 on sun8i is only muxed with mmc0, which makes it a poor choice
for the console. However, some tablets only have pads for uart0
available on the circuit board.

Here we add the uart0 pinmux set for people who need it.

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