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/lib/cpus/aarch64/rainier.S b/lib/cpus/aarch64/rainier.S
index 584ab97..3b7b8b2 100644
--- a/lib/cpus/aarch64/rainier.S
+++ b/lib/cpus/aarch64/rainier.S
@@ -94,7 +94,7 @@
 	bl	errata_n1_1868343_wa
 #endif
 
-#if ENABLE_AMU
+#if ENABLE_FEAT_AMU
 	/* Make sure accesses from EL0/EL1 and EL2 are not trapped to EL3 */
 	mrs	x0, actlr_el3
 	orr	x0, x0, #RAINIER_ACTLR_AMEN_BIT