MdePkg: Use VENDOR_DEVICE_PATH structure for Debug Port device path

MdePkg: Use VENDOR_DEVICE_PATH structure for Debug Port device path

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1229

When converting DebugPort device path from text,
current code use VENDOR_DEFINED_MESSAGING_DEVICE_PATH structure
for Debug port device node.
typedef struct {
  EFI_DEVICE_PATH_PROTOCOL  Header;
  EFI_GUID                  Guid;
  UINT8                     VendorDefinedData[1];
} VENDOR_DEFINED_MESSAGING_DEVICE_PATH;

And Debugport Device Path is a vendor-defined messaging
device path with no data, only a GUID. So it's better to
use VENDOR_DEVICE_PATH to create the Debug port device node.
typedef struct {
  EFI_DEVICE_PATH_PROTOCOL        Header;
  EFI_GUID                        Guid;
} VENDOR_DEVICE_PATH;

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>