dt-bindings: regulator: Add DT bindings for qpnp-lcdb regulator driver

Add devicetree bindings for QTI LCDB regulator driver that helps
with controlling display bias voltage rails for LCD panels.

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

Change-Id: I74852fb4e4447ef96e6416c4ca359ec94f2d2b37
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>