ARM: dts: sun9i: Add Reduced Serial Bus controller device node to A80 dtsi

This patch adds a device node for the Reduced Serial Bus (RSB)
controller and the defacto pinmux setting to the A80 dtsi.

Since there is only one possible pinmux setting for RSB, just
set it in the dtsi.

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