docs: Add SMM Gateway SP security assessment

Add security assessment of the SMM Gateway SP.

Change-Id: I200657f55a7509c44945206fa29c069fe40d140c
Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
2 files changed