dt-bindings: clock: renesas: mstp: Convert to json-schema

Convert the Renesas Clock Pulse Generator (CPG) Module Stop (MSTP)
Clocks Device Tree binding documentation to json-schema.

Drop R-Car Gen2 compatible values, which were obsoleted by the unified
"Renesas Clock Pulse Generator / Module Standby and Software Reset" DT
bindings.
Replace the obsolete example for R-Car H2 by an example that is still
valid.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200508100321.6720-1-geert+renesas@glider.be