Merge rss_comm support.

RSS Comm is a new protocol used to between the SPs and security
subsystems like the Secure Enclave on Corstone-1000 or RSS on Kronos.

This change deprecates the OpenAMP based original solution. This will
be removed in a future release.

Change-Id: Ic9038bd5e185a41ae41c4d95316e7a28eb45adf4
Signed-Off-By: Gyorgy Szing <gyorgy.szing@arm.com>