regulator: labibb: fix standalone mode configuration

Currently, standalone mode is treated as a mode along with other
modes, LCD and AMOLED. Rather than keeping it like that, LCD and
AMOLED mode configurations should be allowed along with the way
LAB and IBB modules are controlled, i.e. standalone or dual.

Remove the standalone mode from the list of modes and keep it as
a configurable parameter via device tree. This way, LCD and
AMOLED modes can be configured along with the way LAB/IBB needs
to be controlled (dual or standalone).

Add support for parent supply to LABIBB device so that LAB and
IBB regulators can vote for MBG when operating in standalone
mode.

CRs-Fixed: 996961
Change-Id: I56882e3a5a01b017e1ba9cd63ab36933a3d469e7
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>