dt-bindings: clock: renesas,cpg-mssr: Document RZ/G2M v3.0 (r8a774a3) clock

dt-bindings: clock: renesas,cpg-mssr: Document RZ/G2M v3.0 (r8a774a3) clock

Add binding documentation for Renesas RZ/G2M v3.0 (a.k.a r8a774a3) Clock
Pulse Generator driver. The r8a774a3 cpg is similar to the r8a774a1 cpg
however, it lacks some modules such as the FCPCI.

Signed-off-by: Oliver Rhodes <oliver.rhodes.aj@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/20240725100534.5374-4-oliver.rhodes.aj@renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>