MdeModulePkg: BaseSerialPortLib16550: Add missing Pcd to UNI

MdeModulePkg: BaseSerialPortLib16550: Add missing Pcd to UNI

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1771

Some buses doesn't allow 8 bit MMIO read/write, this adds support for
32 bits read/write. This patch adds the UNI information on the new Pcd
introduced - PcdSerialRegisterAccessWidth

Signed-off-by: "Tien Hock, Loh" <tien.hock.loh@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: "Zhu, YongHong" <yonghong.zhu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>