feat(fvp): enable FEAT_HCX by default

FEAT_HCX 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_HCX
enabled.

Change-Id: I01aaed15c5a6850176d092b2f0157744fe0a9e13
Signed-off-by: Andre Przywara <andre.przywara@arm.com>