clk: renesas: cpg-mssr: Document R-Car Gen2 support

Document use of the Renesas Clock Pulse Generator / Module Standby and
Software Reset DT Bindings for various member of the R-Car Gen2 family
(H2, M2-W, V2H, M2-N, and E2).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>