dt-bindings: clock: renesas: cpg-mssr: Document r8a774c0

This patch documents RZ/G2E (a.k.a. R8A774C0) bindings for the
Clock Pulse Generator driver.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>