dt-bindings: msm: rpmh-master-stats: Update reg as optional property

This change updates reg as optional property so that same driver
can be used for RPM based targets where profiling unit register
is not present.

This enables rpmh_master_stats driver to read different subsystem
master stats data from SMEM for RPM based targets.

Change-Id: Ib5ccfd5d688fe0f8d69dbdb9c9408ce8970eb576