style(xilinx): replace ARM by Arm in copyrights

The commit 6bb49c876c75 ("style(hooks): adds Arm copyright style fix")
is enforcing proper case for ARM. That's why fix it in plat/xilinx to
make sure that pre-commit.copyright won't be touching platform specific
files.

Change-Id: I49c66e18d46ed871a6aa128c9b2a403d0cf83416
Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/plat/xilinx/zynqmp/include/plat_private.h b/plat/xilinx/zynqmp/include/plat_private.h
index 534777b..9ea052d 100644
--- a/plat/xilinx/zynqmp/include/plat_private.h
+++ b/plat/xilinx/zynqmp/include/plat_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2020, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */