dt-bindings: regulator: Add qpnp-amoled driver bindings

QPNP AMOLED regulator driver supports configuring triple power
supply (OLEDB, AB, IBB) used in AMOLED panels. Add DT bindings
for it.

Change-Id: If19f2e969c848ad276781a61f725eaf656059c30
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>