drivers: mailbox: qcom-rpmh: add RPMH mailbox controller for QCOM SoCs

Add RPMH  mailbox controller driver for QCOM SoC's that have hardware
based RPM communication. The hardware IP known as RSC (Resource State
Coordinator) houses a block that is used to send requests to resources
that are shared with one or more processors in the SoC. The Trigger
Control Set (TCS) is a mailbox slot that can house multiple commands and
when triggered will issue these commands to through an internal bus to
the Resource Power Manager Hardened (RPMH) blocks. The resource state
requests are aggregated along with state requests from other processors
in the SoC and the aggregate value is applied on the resource.

Resource requests made while the CPU is running are called Active state
requests and those sent when the CPU is going into low power mode are
called Sleep state requests. Requests sent during the sleep state of the
processor are matched with Wake state request that restore the state of
the resource to previously requested active levels before the processor
can start executing.

Interrupt responses signal the completion of the requests at the RPMH
and the driver notifies the mailbox framework using the tx_done
callbacks.

This is a snapshot as of 'commit b4a69c05b01172 ("drivers: mailbox:
rpmh-mailbox: rename file to match convention")'.

Change-Id: I6354e9a56dcdb61ab189123233ab469e8d345db3
Signed-off-by: Lina Iyer <ilina@codeaurora.org>