Add tests for FFA function FFA_SPM_ID_GET

Also add the ability for ffa_features_test to only use the expected
return if the minimum FF-A version for the feature is met.

Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
Change-Id: I276500c666365c563a117aa7b250b18f51dcace7
11 files changed