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/common/feat_detect.c b/common/feat_detect.c
index 49b5360..57d6ae0 100644
--- a/common/feat_detect.c
+++ b/common/feat_detect.c
@@ -204,6 +204,8 @@
 		      "S1POE", 1, 1);
 	check_feature(ENABLE_FEAT_MTE_PERM, read_feat_mte_perm_id_field(),
 		      "MTE_PERM", 1, 1);
+	check_feature(ENABLE_FEAT_CSV2_3, read_feat_csv2_id_field(),
+		      "CSV2_3", 3, 3);
 
 	/* v9.0 features */
 	check_feature(ENABLE_BRBE_FOR_NS, read_feat_brbe_id_field(),