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/Makefile b/Makefile
index 87ff22e..3536763 100644
--- a/Makefile
+++ b/Makefile
@@ -1256,6 +1256,7 @@
 	ENABLE_FEAT_AMU \
 	ENABLE_FEAT_AMUv1p1 \
 	ENABLE_FEAT_CSV2_2 \
+	ENABLE_FEAT_CSV2_3 \
 	ENABLE_FEAT_DIT \
 	ENABLE_FEAT_ECV \
 	ENABLE_FEAT_FGT \
@@ -1411,6 +1412,7 @@
 	ENABLE_FEAT_SEL2 \
 	ENABLE_FEAT_VHE \
 	ENABLE_FEAT_CSV2_2 \
+	ENABLE_FEAT_CSV2_3 \
 	ENABLE_FEAT_PAN \
 	ENABLE_FEAT_TCR2 \
 	ENABLE_FEAT_S2PIE \