dt-bindings: power: Add r8a77961 SYSC power domain definitions

Add power domain indices for the R-Car M3-W+ (R8A77961) SoC.

Based on Rev. 2.00 of the R-Car Series, 3rd Generation, Hardware User’s
Manual (Jul. 31, 2019).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20191023122911.12166-6-geert+renesas@glider.be