dt-bindings: power: renesas,rcar-sysc: Document R-Car V4M support

Document support for the System Controller (SYSC) in the R-Car V4M
(R8A779H0) SoC.

Signed-off-by: Duy Nguyen <duy.nguyen.rh@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/6315cbd0b6e9b92a7914d98f397a2c663ad521c6.1706194617.git.geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>