dt-bindings: pwm: Convert PWM bindings to json-schema

Convert generic PWM controller bindings to DT schema format using
json-schema.  The consumer bindings are provided by dt-schema.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>