dt-bindings: clock: renesas,cpg-mssr: Document R-Car V4M support

Document support for the Clock Pulse Generator (CPG) and Module Standby
Software Reset (MSSR) module on the Renesas R-Car V4M (R8A779H0) SoC.

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/eb3cd02b62f3ca834df079a3f1e551d9414fe42a.1706194617.git.geert+renesas@glider.be