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

Convert the tiehrpwm binding to DT schema format using json-schema.
Along with this conversion the following changes are included:
- 'clock' and 'clock-names' properties are marked as required as
   driver fails to probe without these properties
- Dropped ti,am33xx-ehrpwm as it is no longer applicable.
- 'power-domains' property is introduced and marked as optional.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>