feat(fvp): enable FEAT_FGT by default

feat(fvp): enable FEAT_FGT by default

FEAT_FGT is one of the features for which Linux necessarily requires EL3
enablement, when the feature is present on a PE.

To cover the effect of different FVP command line parameters, include
the feature into the standard FVP build, but use FEAT_STATE_CHECK, to
always do runtime checks before accessing feature specific registers.

This prevents a Linux crash when the FVP is called with FEAT_FGT
enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I55fbb2706aefbc3ab67c476e3f8b6ea74ae0d66c