feat(cpufeat): add feature detection for FEAT_CSV2_3

This feature provides support to context save the
SCXTNUM_ELx register. FEAT_CSV2_3 implies the implementation
of FEAT_CSV2_2. FEAT_CSV2_3 is supported in AArch64 state only
and is an optional feature in Arm v8.0 implementations.

This patch adds feature detection for v8.9 feature FEAT_CSV2_3,
adds macros for ID_AA64PFR0_EL1.CSV2 bits [59:56] for detecting
FEAT_CSV2_3 and macro for ENABLE_FEAT_CSV2_3.

Change-Id: Ida9f31e832b5f11bd89eebd6cc9f10ddad755c14
Signed-off-by: Sona Mathew <sonarebecca.mathew@arm.com>
diff --git a/plat/arm/board/fvp/platform.mk b/plat/arm/board/fvp/platform.mk
index df26dd7..eef06d4 100644
--- a/plat/arm/board/fvp/platform.mk
+++ b/plat/arm/board/fvp/platform.mk
@@ -75,6 +75,7 @@
 endif
 ENABLE_SYS_REG_TRACE_FOR_NS	:= 2
 ENABLE_FEAT_CSV2_2		:= 2
+ENABLE_FEAT_CSV2_3		:= 2
 ENABLE_FEAT_DIT			:= 2
 ENABLE_FEAT_PAN			:= 2
 ENABLE_FEAT_MTE_PERM		:= 2