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.

Change-Id: I0d7b152dbe20d9336fae8bf7d9a2f84ff569fb13
Signed-off-by: David Collins <collinsd@codeaurora.org>