dt-bindings: soc: renesas: Document R-Car V4M Gray Hawk Single

Document the compatible values for the Renesas R-Car V4M (R8A779H0) SoC
and the Renesas Gray Hawk Single development board.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/1065a81afb8dbed09b0098b51dc7fb22da421a97.1706194617.git.geert+renesas@glider.be