dt-bindings: clock: renesas: cpg-mssr: Convert to json-schema

Convert the Renesas Clock Pulse Generator / Module Standby and Software
Reset Device Tree binding documentation to json-schema.

Note that #reset-cells was incorrecty marked a required property for
RZ/A2 before.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200303094848.23670-1-geert+renesas@glider.be