product: optee: declare CFG_SCPFW_MOD_xxx CMake directives

Declare CFG_SCPFW_MOD_xxx CMake directives for OP-TEE to tell
SCP-firmware CMake configuration process which modules are
to be embedded. This allows a product (like optee-stm32mp1) to
support several variants based on different configuration settings.

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