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/make_helpers/defaults.mk b/make_helpers/defaults.mk
index b928fcf..ddf81e6 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -133,9 +133,6 @@
 # Use BRANCH_PROTECTION to enable PAUTH.
 ENABLE_PAUTH			:= 0
 
-# Flag to enable access to the HAFGRTR_EL2 register
-ENABLE_FEAT_AMUv1		:= 0
-
 # Flag to enable AMUv1p1 extension.
 ENABLE_FEAT_AMUv1p1		:= 0
 
@@ -367,7 +364,7 @@
 # enabled at ELX.
 CTX_INCLUDE_MTE_REGS		:= 0
 
-ENABLE_AMU			:= 0
+ENABLE_FEAT_AMU			:= 0
 ENABLE_AMU_AUXILIARY_COUNTERS	:= 0
 ENABLE_AMU_FCONF		:= 0
 AMU_RESTRICT_COUNTERS		:= 0