clk: renesas: cpg-mssr: Add r8a774a1 support

Add RZ/G2M (R8A774A1) Clock Pulse Generator / Module Standby and Software
Reset support.

Based on the Table 8.2b of "RZ/G Series, 2nd Generation User's Manual:
Hardware ((Rev. 0.61, June 12, 2018)".

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