product: optee-stm32mp1: may embed MSG_SMT

Product optee-stm32mp1 needs to embed module MSG_SMT instead of
OPTEE_SMT when the platform use OP-TEE shared memory instead of
a piece of SRAM for SCMI communication.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>