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