dt-bindings: regulator: add refgen regulator bindings

Some Qualcomm Technologies, Inc. SoCs such as SDM845 utilize
reference bias generators for various internal PHY blocks.
These are called REFGENs.  Add binding documentation for
REFGEN devices.

This is a snapshot from msm-4.19
commit e5cf0a03c6aa ("dt-bindings: regulator: add refgen
regulator bindings").

Change-Id: I83c37e3a1fb33bd552849f7cf20d602a901a40d2