docs(build): describes the SPMC_OPTEE build option

Explains that the SPMC_OPTEE build option is used to load the SPMC at
S-EL1 using an OP-TEE specific mechanism.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Change-Id: I71757d2d9ac98caf0ac6d8e64b221adaa0f70846
1 file changed