dt-bindings: regulator: add gdsc-regulator bindings

Globally distributed switch controllers (GDSCs) are used to
switch the power supplied to a particular hardware block within
a Qualcomm Technologies, Inc. SoC on and off.  Add gdsc-regulator
device bindings.

Change-Id: Iae47c4fe22c19060d120ef51249b15b20160e0fe