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

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

This snapshot is taken as of msm-4.14
commit 9bb584ae ("msm/sde/rotator: Add rev checks for sdmmagpie").

Change-Id: I7831b2c2c380309d04f2367b9a2818b7e0805f2e
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>