FVP: Always assume shifted affinity with MT bit

FVP: Always assume shifted affinity with MT bit

At present, the MPIDR validation on FVP relies on MT bit set along
with shifted affinities. This currently is additionally dependent
on the FVP model being of variant C. This however should be based
on the presence of MT bit alone.

This patch makes the change to always assume that the affinities
are shifted in the FVP model when MT bit is present.

Change-Id: I09fcb0126e1b38d29124bdeaf3450a60b95d485d
Signed-off-by: Isla Mitchell <isla.mitchell@arm.com>