tftf: spci: probe if SPMC is OP-TEE at S-EL1

For pre-Armv8.4 platform, the supported model is that SPMC runs at
S-EL1. The candidate is OP-TEE with SPCI protocol adaptation. This patch
performs a version check through direct messaging and determines if
OP-TEE runs as SPMC by checking version responses. If this does not
match, then it assumes SPMC runs at S-EL2 with two Cactus instances
running at S-EL1.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I34194b6e8b3d447a25f2153bbac1be2631cd7a5e