drivers: qcom: Add standalone mode support for RPMH

Allow RPMH requests to succeed even when the Always-On subsystem is not
available. In standalone mode, requests are never sent to the TCS
mailbox driver. The command DB provides the information to make the
decision to run in standalone mode or not.

Change-Id: Ia2bc5024da0d7e6607347b1a481b2feb9584059a
Signed-off-by: Lina Iyer <ilina@codeaurora.org>