greybus: pwm: Add runtime_pm support

Add runtime pm support for the pmw driver.

Testing Done: Set the parameters of pwm0, and enable.
Disable pwm0 and let the module enter standby.
Enable pwm0, and observe that with an oscilloscope
that the wave form is the same as before.

Signed-off-by: Axel Haslam <haslam_axel@projectara.com>
Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Alex Elder <elder@linaro.org>