MdePkg: Add UEFI Unaccepted memory definition

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3937

Plase refer to:
  UEFI Spec v2.9 Table 7-5 Memory Type Usage before ExitBootServices()

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Min Xu <min.m.xu@intel.com>

MdePkg/Include/Uefi/UefiMultiPhase.h

@@ -103,6 +103,11 @@ typedef enum {
   /// however it happens to also support byte-addressable non-volatility.
   ///
   EfiPersistentMemory,
+  ///
+  /// A memory region that describes system memory that has not been accepted
+  /// by a corresponding call to the underlying isolation architecture.
+  ///
+  EfiUnacceptedMemoryType,
   EfiMaxMemoryType
 } EFI_MEMORY_TYPE;