regulator: qpnp-labibb: Add support for notifier callback

Some drivers (e.g. OLEDB) would need to know about LAB_VREG_OK
status from the LABIBB module. Hence, add support for the
notifier callback so that required drivers can be notified about
LAB_VREG_OK status.

Change-Id: Ib60c94c7557ee6ffb7595dee5bd268bb76faaf6e
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>