drivers: qcom: rpmh: add RPMH utility functions for TCS communication

Drivers that communicate with the TCS mailbox driver duplicate the same
functionality in managing memory for requests, threaded handling of
completion responses etc. Collect all such common functionality into a
simple set of API that abstracts complexity and optimizations in
communication with the mailbox controller.

The key benefits of using this API instead of directly using mailbox API
for QTI drivers are -
	* Caching of sleep and wake requests
	* Handling of completion callbacks
	* Memory management to avoid fragmenting kernel memory
	* Batch processing of requests

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

Change-Id: Id80b9312ff14537f9732acfa090f8e997158b63a
Signed-off-by: Lina Iyer <ilina@codeaurora.org>