Implements Ocall2 sequence for MSG SCMI channels. An MSG SCMI channel is
an SCMI channel using OP-TEE native shared memory. Agent (Linux)
passes the SCM shm reference when invoking SCMI PTA to create an Ocall2
threaded context (PTA command PTA_SCMI_CMD_OCALL2_MSG_THREAD) instead
of using a statically allocated and mapped IOMEM shred memory buffer.

Ocall2 second parameter #2 is used to pass the SCMI message size in
receive and transmit communication.

Change-Id: I7afaaf3a3d0329942a8d0d41afc840095b1ce3c1
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/optee/optee_os/+/298720
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Gatien CHEVALLIER <gatien.chevallier@st.com>
Domain-Review: Lionel DEBIEVE <lionel.debieve@foss.st.com>