dt-bindings: pwm: google,cros-ec: include generic pwm schema

Include generic pwm.yaml schema, which enforces PWM node naming.  Keep
the old name in bindings as deprecated.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>