ARM: dts: sun9i: cubieboard4: Enable Reduced Serial Bus controller

The Reduced Serial Bus (RSB) controller is used to talk to the 3
companion ICs (2 PMICs, 1 RTC/codec IC) on the board.

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