dt-bindings: clock: Convert pwm-clock to DT schema

Convert the pwm-clock binding to DT schema format. A straight-forward
conversion.

Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221011162919.3025038-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>