refactor(amu): unify ENABLE_AMU and ENABLE_FEAT_AMUv1

So far we have the ENABLE_AMU build option to include AMU register
handling code for enabling and context switch. There is also an
ENABLE_FEAT_AMUv1 option, solely to protect the HAFGRTR_EL2 system
register handling. The latter needs some alignment with the new feature
scheme, but it conceptually overlaps with the ENABLE_AMU option.

Since there is no real need for two separate options, unify both into a
new ENABLE_FEAT_AMU name in a first step. This is mostly just renaming at
this point, a subsequent patch will make use of the new feature handling
scheme.

Change-Id: I97d8a55bdee2ed1e1509fa9f2b09fd0bdd82736e
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/common/feat_detect.c b/common/feat_detect.c
index 9218c07..ba8c82c 100644
--- a/common/feat_detect.c
+++ b/common/feat_detect.c
@@ -187,7 +187,7 @@
 
 	/* v8.4 features */
 	read_feat_dit();
-	check_feature(ENABLE_FEAT_AMUv1, read_feat_amu_id_field(),
+	check_feature(ENABLE_FEAT_AMU, read_feat_amu_id_field(),
 		      "AMUv1", 1, 2);
 	check_feature(ENABLE_MPAM_FOR_LOWER_ELS, read_feat_mpam_version(),
 		      "MPAM", 1, 17);