MdePkg/DxeRuntimeDebugLibSerialPort: Add new APIs

MdePkg/DxeRuntimeDebugLibSerialPort: Add new APIs

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1395

Add new APIs' implementation (DebugVPrint, DebugBPrint)
in the DebugLib instance. These APIs would expose print
routines with VaList parameter and BaseList parameter.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>