dt-bindings: clock: renesas: cpg-mssr: Document r8a774b1 binding

Add binding documentation for the RZ/G2N (R8A774b1) Clock Pulse
Generator driver.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1568881036-4404-6-git-send-email-biju.das@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>