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

Change-Id: I01aaed15c5a6850176d092b2f0157744fe0a9e13
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/arm/board/fvp/platform.mk b/plat/arm/board/fvp/platform.mk
index 2f8284d..efbf68f 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -465,6 +465,7 @@
 # Linux relies on EL3 enablement if those features are present
 ifeq (${SPMC_AT_EL3}, 1)
 PLAT_BL_COMMON_SOURCES	+=	plat/arm/board/fvp/fvp_el3_spmc.c