feat(trf): enable trace filter control register access from lower NS EL

Introduced a build flag 'ENABLE_TRF_FOR_NS' to enable trace filter
control registers access in NS-EL2, or NS-EL1 (when NS-EL2 is
implemented but unused).

Change-Id: If3f53b8173a5573424b9a405a4bd8c206ffdeb8c
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/include/lib/extensions/trf.h b/include/lib/extensions/trf.h
new file mode 100644
index 0000000..18f17f3
--- /dev/null
+++ b/include/lib/extensions/trf.h
@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef TRF_H
+#define TRF_H
+
+void trf_enable(void);
+
+#endif /* TRF_H */