mfd: pm8921: change PM8921 core and regulator init to postcore

The pm8921-core and pm8921-regulator drivers are currently registered
in the subsys init level.  This is not soon enough because the
acpuclock driver needs them to be available during the arch init
level.  Therefore, change them to postcore init so that they are
ready in time.

Change-Id: I6ec7d42e0ef459e7f76621ce518cf9a9c9093c3c
Signed-off-by: David Collins <collinsd@codeaurora.org>