greybus: lights: Add runtime pm support

Modify Lights greybus driver to support runtime PM framework.
The suspend and resume function have been tested with gpbridge-test
image by sysfs. Lights functions work well on suspend/resume.

Testing Done: Compiled and verified on EVT2 and gpbridge-test module
              with device class daughter board.

Signed-off-by: Kris Huang <huang_kris@projectara.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>