MdePkg: Add ACPI 6.4 header file

Bugzilla: 3516 (https://bugzilla.tianocore.org/show_bug.cgi?id=3516)

This patch introduces a header file for the ACPI specification version
6.4. Currently it is based on the Acpi63.h header file however makes
six changes:

1. Replace all occurences of "6_3"/"6.3" with "6_4/6.4".
2. Remove the trailing underscore from the header guard in accordance
   with the EDK2 coding standards, section 5.3.5.
3. Make Acpi64.h the latest ACPI definition included by Acpi.h.
4. Fix the BGRT Status field comment to match the ACPI 6.3A spec.
5. Fix several typos where definitions were named "PMMT" when it should
   be "PMTT".
6. Fix a typo: "PPTT Platform Communication Channel" should be "PDTT
   Platform Communication Channel".

Signed-off-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

MdePkg/Include/IndustryStandard/Acpi.h

@@ -3,7 +3,7 @@
   consumed by drivers that do not care about ACPI versions.
 
   Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>
-  Copyright (c) 2019, ARM Ltd. All rights reserved.<BR>
+  Copyright (c) 2019 - 2021, ARM Ltd. All rights reserved.<BR>
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -11,6 +11,6 @@
 #ifndef _ACPI_H_
 #define _ACPI_H_
 
-#include <IndustryStandard/Acpi63.h>
+#include <IndustryStandard/Acpi64.h>
 
 #endif