docs: Add Attestation SP security assessment

Add security assessment of the Attestation SP.

Change-Id: I7ba5f324d13b159a9d3ccef9267f2d90dc16166a
Signed-off-by: Gabor Toth <gabor.toth2@arm.com>
Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>
2 files changed