dt-bindings: pwm: Convert google,cros-ec-pwm.txt to YAML format

Convert the binding file google,cros-ec-pwm.txt to YAML format.

This was tested and verified on ARM64 with:

make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.yaml
make dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/pwm/google,cros-ec-pwm.yaml

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>