DynamicTablesPkg: Fix serial port namespace path in DBG2

DynamicTablesPkg: Fix serial port namespace path in DBG2

According to the Debug Port Table 2 (DBG2) specification,
February 17, 2021, the NamespaceString is a NULL terminated
ASCII string that consists of a fully qualified reference
to the object that represents the serial port device in the
ACPI namespace.

The DBG2 table generator did not populate the full device
path for the serial port device, and this results in a FWTS
test failure.

Therefore, populate the full namespace device path for the
serial port in DBG2 table.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Tested-by: Jagadeesh Ujja <Jagadeesh.Ujja@arm.com>
Tested-by: Sunny Wang <sunny.wang@arm.com>