feat(fvp): mock support for CCA NV ctr

AEM FVP does not have a third CCA NV counter so the
implementation will fake it by returning the Trusted
NV counter value when the caller requests the CCA NV
counter. This allows us to use the CCA CoT on AEM FVP
nonetheless.

The FVP platform port now gets its own version of
plat_get_nv_ctr() as it now need to diverge from the
common implementation provided at the Arm development
platforms level.

Change-Id: I3258f837249a539d943d6d783406ba222bd4554e
Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>