clk: renesas: cpg-mssr: Add common R-Car Gen2 support

Add the common R-Car Gen2 (and RZ/G) Clock Pulse Generator / Module
Standby and Software Reset support code, using the CPG/MSSR driver
core.

Based on the proof-of-concept R8A7791 CPG/MSSR patch by Geert
Uytterhoeven <geert+renesas@glider.be>.

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