drivers: mailbox: add TCS Mailbox driver for RPMH communication

Qualcomm Technologies SoCs that have H/W accelerators for shared
resources uses a mailbox communication model to issue shared resource
requests. Triggered Command Set (TCS) is the H/W for such communication.
Each TCS controller block provides slots for multiple commands. The
controller driver picks the right slot for the request (depending on the
requested resource state) and issues the request to the accelerator I/P
blocks.

Change-Id: If244398ca61a373af133923a5f9d5af044829a5d
Signed-off-by: Lina Iyer <ilina@codeaurora.org>