ArmPkg: Fix Ecc error 8003

This patch fixes the following Ecc reported error:
The #ifndef at the start of an include file should have
one postfix underscore, and no prefix underscore character

Some include guards have been modified to match the name of the
header file. Some comments have also been added on the closing
'#endif'.

Cc: Bret Barkelew <bret.barkelew@microsoft.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

ArmPkg/Drivers/CpuDxe/CpuDxe.h

@@ -7,8 +7,8 @@
 
 **/
 
-#ifndef __CPU_DXE_ARM_EXCEPTION_H__
-#define __CPU_DXE_ARM_EXCEPTION_H__
+#ifndef CPU_DXE_H_
+#define CPU_DXE_H_
 
 #include <Uefi.h>
 
@@ -143,4 +143,4 @@ SetGcdMemorySpaceAttributes (
   IN UINT64                              Attributes
   );
 
-#endif // __CPU_DXE_ARM_EXCEPTION_H__
+#endif // CPU_DXE_H_