feat(sfcp): add SFCP stack and PSA call

Add SFCP stack from trusted-firmware-m commit 8eb72a3bc5cc. SFCP is the
Simple Firmware Communication Protocol, which is a more substantial
software stack designed to replace the existing RSE comms (and indeed
wider communication between firmware components in the system). It has
support for both polling mode and interrupt driver communication
handling, and is able to support any underlying transport (this patch
adds MHU only). It requires a static routing layout between system
components.

This patch adds the link layer (with support for the MHU transport),
top-level SFCP API implementation and the implementation of PSA
call making use of the SFCP API.

Note that encryption support is not implemented and only the stub
encryption implementation is added in this patch. This can be
implemented when TF-A needs it.

The sfcp_link_hal.c implementation is the same as that in
trusted-firmware-m, and it makes use of the MHU V2 and V3 drivers
directly. This is possible as the underlying MHU driver APIs is the same
in trusted-firmware-m and trusted-firmware-a.

Change-Id: I2318ea4bdb4e533b8a4a5000040aec0635a83857
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>