refactor(cpufeat): convert FEAT_HCX to new scheme

Use the generic check function in feat_detect.c, and split the feature
check into two functions, as done for FEAT_FGT before.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I0a4f973427c10d5d15c414ff5e12b18b7e645fae
diff --git a/bl31/bl31_main.c b/bl31/bl31_main.c
index 2a3d838..a4640b1 100644
--- a/bl31/bl31_main.c
+++ b/bl31/bl31_main.c
@@ -99,7 +99,7 @@
 	 * an exception would occur during context save/restore if enabled but
 	 * not supported.
 	 */
-	assert(is_feat_hcx_present());
+	assert(is_feat_hcx_supported());
 #endif /* ENABLE_FEAT_HCX */
 
 #if CTX_INCLUDE_PAUTH_REGS