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

Convert the Renesas Clock Pulse Generator (CPG) Device Tree
binding documentation to json-schema, combining support for:
  - R-Mobile APE6 (R8A73A4) and A1 (R8A7740),
  - R-Car M1 (R8A7778) and H1 (R8A7779),
  - RZ/A1 (R7S72100),
  - SH-Mobile AG5 (SH73A0).

Keep the example for R-Mobile A1, which shows most properties.
Drop the consumer examples, as they do not belong here.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200518081644.23683-1-geert+renesas@glider.be