ARM / shmobile: Use genpd_queue_power_off_work()

Make pd_power_down_a3rv() use genpd_queue_power_off_work() to queue
up the powering off of the A4LC domain to avoid queuing it up when
it is pending.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Magnus Damm <damm@opensource.se>