feat: add register definitions needed for SMCCC_ARCH_FEATURE_AVAILABILITY

Also slightly optimised some redundant feature functions

Co-developed-by: Charlie Bareham <charlie.bareham@arm.com>
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I6dcc11060a2f3697a8aa41443e9cfc665b2b7c74
diff --git a/tftf/tests/extensions/amu/test_amu.c b/tftf/tests/extensions/amu/test_amu.c
index 21305a7..0cc4395 100644
--- a/tftf/tests/extensions/amu/test_amu.c
+++ b/tftf/tests/extensions/amu/test_amu.c
@@ -1,11 +1,12 @@
 /*
- * Copyright (c) 2018-2021, Arm Limited. All rights reserved.
+ * Copyright (c) 2018-2024, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include <amu.h>
 #include <arch.h>
+#include <arch_features.h>
 #include <arch_helpers.h>
 #include <assert.h>
 #include <debug.h>