refactor(cpufeat): align FEAT_SEL2 to new feature handling

In ARMv8.4, the EL2 exception level got added to the secure world.
Adapt and rename the existing is_armv8_4_sel2_present() function, to
align its handling with the other CPU features.

Change-Id: If11e1942fdeb63c63f36ab9e89be810347d1a952
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 ef62e73..d9cca5e 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -474,6 +474,7 @@
 ENABLE_FEAT_CSV2_2		:= 2
 ENABLE_FEAT_ECV			:= 2
 ENABLE_FEAT_PAN			:= 2
+ENABLE_FEAT_SEL2		:= 2
 ENABLE_FEAT_TWED		:= 2
 ENABLE_FEAT_VHE			:= 2
 ENABLE_MPAM_FOR_LOWER_ELS	:= 2