bindings: spmi: add QTI Glink SPMI debug controller binding documentation

Add binding documentation for the Qualcomm Technologies, Inc.
Glink SPMI debug controller device.  This device provides an
interface to read and write PMIC registers over PMIC Glink using
a remote subsytem (e.g. DSP).  This allows for debugging PMIC
peripherals that would typically only be accessible to the
charger and fuel gauging firmware running on the remote
subsystem.

Change-Id: I1779c7841dfed8e49c07705d6803b7aad196c4d4