Convert pwrdm_mutex to pwrdm_rwsem to avoid trying to relock mutex in the
event that the pwrdm_for_each() callback function calls something that
triggers a pwrdm_lookup().  Problem found by Jouni Högander
<jouni.hogander@nokia.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>