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/mediatek/common/common_config.mk b/plat/mediatek/common/common_config.mk
index 851eb2c..31a61e0 100644
--- a/plat/mediatek/common/common_config.mk
+++ b/plat/mediatek/common/common_config.mk
@@ -19,7 +19,7 @@
 ENABLE_STACK_PROTECTOR := strong
 # AMU, Kernel will access amuserenr_el0 if PE supported
 # Firmware _must_ implement AMU support
-ENABLE_AMU := 1
+ENABLE_FEAT_AMU := 2
 VENDOR_EXTEND_PUBEVENT_ENABLE := 1
 
 # MTK define options