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/Include/Library/StandaloneMmMmuLib.h

@@ -6,8 +6,8 @@
 
 **/
 
-#ifndef __STANDALONEMM_MMU_LIB__
-#define __STANDALONEMM_MMU_LIB__
+#ifndef STANDALONE_MM_MMU_LIB_
+#define STANDALONE_MM_MMU_LIB_
 
 EFI_STATUS
 ArmSetMemoryRegionNoExec (
@@ -33,4 +33,4 @@ ArmClearMemoryRegionReadOnly (
   IN  UINT64                    Length
   );
 
-#endif /* __STANDALONEMM_MMU_LIB__ */
+#endif /* STANDALONE_MM_MMU_LIB_ */