firmware: arm_scmi: sync with OP-TEE implementation

Sync with SCMI OP-TEE transport reviewed at upstream through pull-request
linaro-swg/linux#90.

Change-Id: Iae3a0e44717983bcd6db126edda2ea4bedf9fe7d
Signed-off-by: Etienne Carriere <[email protected]>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/202488
Reviewed-by: Etienne CARRIERE <[email protected]>
Reviewed-by: MPUOSTL
Tested-by: Alexandre TORGUE <[email protected]>

Author: etienne-lms