feat(bl): add interface to query TF-A semantic ver

Adding interface for stand-alone semantic version of TF-A
for exporting to RSS attestation, and potentially other areas
as well.

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: Ib4a2c47aa1e42a3b850185e674c90708a05cda53