chore: update to use Arm word across TF-A

Align entire TF-A to use Arm in copyright header.

Change-Id: Ief9992169efdab61d0da6bd8c5180de7a4bc2244
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
diff --git a/lib/libc/strnlen.c b/lib/libc/strnlen.c
index b944e95..49e9f7a 100644
--- a/lib/libc/strnlen.c
+++ b/lib/libc/strnlen.c
@@ -27,7 +27,7 @@
  */
 
 /*
- * Portions copyright (c) 2018, ARM Limited and Contributors.
+ * Portions copyright (c) 2018, Arm Limited and Contributors.
  * All rights reserved.
  */