clk: renesas: cpg-mssr: Add R8A77970 support

Add R-Car V3M (R8A77970) Clock Pulse Generator / Module Standby and
Software Reset support, using the CPG/MSSR driver core and the common
R-Car Gen3 code.

Based on the original (and large) patch by Daisuke Matsushita
<daisuke.matsushita.ns@hitachi.com>.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>