feat(plat/qemu): add SPMD support with SPMC at S-EL1

Adds support for SPMD with SPMC at S-EL1. A new config option SPMC_OPTEE
is added to support loading the special OP-TEE images when configured
with SPD=spmd. With or without SPMC_OPTEE. It should still be possible
to load another BL32 payload implementing a SPMC, provided that entry
point is the same as load address, that is, BL32_BASE.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Change-Id: Ie61dcd1ee564688baee1b575030e63dc2bb85121