refactor(amu): use new AMU feature check routines

The AMU extension code was using its own feature detection routines.
Replace them with the generic CPU feature handlers (defined in
arch_features.h), which get updated to cover the v1p1 variant as well.

Change-Id: I8540f1e745d7b02a25a6c6cdf2a39d6f5e21f2aa
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/include/lib/extensions/amu.h b/include/lib/extensions/amu.h
index 6452f7e..de476e4 100644
--- a/include/lib/extensions/amu.h
+++ b/include/lib/extensions/amu.h
@@ -14,11 +14,23 @@
 
 #include <platform_def.h>
 
+#if ENABLE_FEAT_AMU
 #if __aarch64__
 void amu_enable(bool el2_unused, cpu_context_t *ctx);
 #else
 void amu_enable(bool el2_unused);
 #endif
+#else
+#if __aarch64__
+static inline void amu_enable(bool el2_unused, cpu_context_t *ctx)
+{
+}
+#else
+static inline void amu_enable(bool el2_unused)
+{
+}
+#endif
+#endif
 
 #if ENABLE_AMU_AUXILIARY_COUNTERS
 /*