feat(firme): introduce FIRME attestation service

Add attestation service and support for `FIRME_ATTEST_PAT_GET`. This ABI
is used to enable retrieval of platform attestation tokens.

Change-Id: Iea086eb5cd516d5d8f7dbf78a786b3099fe9b064
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>