regulator: qpnp-oledb: Add lab_vreg_ok notifier callback

OLEDB driver needs the LAB_VREG_OK status to program the PD_CTL
register after the oledb module is enabled. As this status register
is not accessible from the OLEDB driver, register a notifier callback
with LABIBB driver to get LAB_VREG_OK status.

Change-Id: I85009688a2accb6246135d22e08ee21fb8296f62
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>