dt-bindings: regulator: Add DT bindings for qpnp-labibb regulator

Add device tree bindings for LAB/IBB regulator that supports the
display bias voltage rails on QTI PMICs that powers up LCD and
AMOLED panels.

This snapshot is taken as of msm-4.14
commit 9bb584ae3a9d ("msm/sde/rotator: Add rev checks for sdmmagpie").

Change-Id: I620f21301e897a87405a30851af9c580a24aa4b1
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>