Use SET_SYSTEM_SLEEP_PM_OPS to simplify code.

Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>
Acked-by: NGuennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: NChris Ball <cjb@laptop.org>