ARM: sun8i: a83t: Add device node and pinmux setting for RSB controller

The A83T has an RSB controller for talking to the PMIC and audio codec.
Add a device node for it. Since there is only one usable pinmux setting,
for it, add that as well.

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