ARM: dts: sun8i: Add pin muxing option for R_UART

R_UART is available on extra pads on certain tablets, which makes it
ideal for use as a console. Here we add the pins for it.

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