feat(sme): enable SME2 functionality for NS world

feat(sme): enable SME2 functionality for NS world

FEAT_SME2 is an extension of FEAT_SME and an optional feature
from v9.2. Its an extension of SME, wherein it not only
processes matrix operations efficiently, but also provides
outer-product instructions to accelerate matrix operations.
It affords instructions for multi-vector operations.
Further, it adds an 512 bit architectural register ZT0.

This patch implements all the changes introduced with FEAT_SME2
to ensure that the instructions are allowed to access ZT0
register from Non-secure lower exception levels.

Additionally, it adds support to ensure FEAT_SME2 is aligned
with the existing FEATURE DETECTION mechanism, and documented.

Change-Id: Iee0f61943304a9cfc3db8f986047b1321d0a6463
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>