dt-bindings: clock: renesas: cpg-mssr: Document r8a7744 binding

Add binding documentation for the RZ/G1N (R8A7744) Clock Pulse
Generator driver.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>