dt-bindings: clock: gpio-mux-clock: Convert to json-schema

Convert the simple GPIO clock multiplexer Device Tree binding
documentation to json-schema.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/14cb3b4da446f26a4780e0bd1b58788eb6085d05.1623414619.git.geert+renesas@glider.be


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>