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/plat/arm/board/rdv1mc/platform.mk b/plat/arm/board/rdv1mc/platform.mk
index df0b09a..92f7c10 100644
--- a/plat/arm/board/rdv1mc/platform.mk
+++ b/plat/arm/board/rdv1mc/platform.mk
@@ -68,7 +68,7 @@
 $(eval $(call TOOL_ADD_PAYLOAD,${NT_FW_CONFIG},--nt-fw-config,${NT_FW_CONFIG}))
 
 override CTX_INCLUDE_AARCH32_REGS	:= 0
-override ENABLE_AMU			:= 1
+override ENABLE_FEAT_AMU		:= 1
 
 ifneq ($(CSS_SGI_PLATFORM_VARIANT),0)
  $(error "CSS_SGI_PLATFORM_VARIANT for RD-V1-MC should always be 0, \