msm: ocmem: Add support for low power clients

Add support for low power clients to map and unmap OCMEM buffers.
These operations trigger RDM transfers using the BR/DM core which
enables low power clients to migrate segments between main memory and
OCMEM.

Also add the required APIs for supporting eviction and
restoration of buffers.

Change-Id: I6b641510f8ef27da2b347d13af468a1ed6664c9e
Signed-off-by: Naveen Ramaraj <nramaraj@codeaurora.org>