MdeModulePkg Xhci: Handle value 5 in Port Speed field of PORTSC

MdeModulePkg Xhci: Handle value 5 in Port Speed field of PORTSC

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1267

The value 5 Port Speed field of PORTSC is new defined in
XHCI 1.1 spec November 2017.

This patch updates XhciDxe and XhciPei to handle it, otherwise
the USB 3.1 device may not be recognized with the XHCI controller
following XHCI 1.1 spec November 2017.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>