ArmPkg: Fix Ecc error 8001 in Chipset

This patch fixes the following Ecc reported error:
Only capital letters are allowed to be used
for #define declarations

Edk2 coding standard stating that:
"Names starting with one or two underscores, such as
_MACRO_GUARD_FILE_NAME_H_, must not be used."
the include guard of ArmCortexA5x.h is also updated.

Ref:
https://edk2-docs.gitbook.io/edk-ii-c-coding-standards-specification/
5_source_files/53_include_files#
5-3-5-all-include-file-contents-must-be-protected-by-a-include-guard

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Include/Chipset/ArmCortexA5x.h

@@ -1,13 +1,13 @@
 /** @file
 
-  Copyright (c) 2012-2014, ARM Limited. All rights reserved.
+  Copyright (c) 2012 - 2021, Arm Limited. All rights reserved.<BR>
 
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
 
-#ifndef __ARM_CORTEX_A5x_H__
-#define __ARM_CORTEX_A5x_H__
+#ifndef ARM_CORTEX_A5X_H_
+#define ARM_CORTEX_A5X_H_
 
 //
 // Cortex A5x feature bit definitions
@@ -41,4 +41,4 @@ ArmUnsetCpuExCrBit (
   IN  UINT64    Bits
   );
 
-#endif
+#endif // ARM_CORTEX_A5X_H_