Document SoC specific bindings for R-Car RZ/G1C(r8a77470) SoC.

Signed-off-by: Cao Van Dong <cv-dong@jinso.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>