MdeModulePkg/XhciDxe: Add access xHCI Extended Capabilities Pointer

Add support process Port Speed field value of PORTSC according to
Supported Protocol Capability (define in xHCI spec 1.1)

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3914

The value of Port Speed field in PORTSC bit[10:13]
(xHCI spec 1.1 section 5.4.8) should be change to use this value to
query thru Protocol Speed ID (PSI) (xHCI spec 1.1 section 7.2.1)
in xHCI Supported Protocol Capability and return the value according
the Protocol Speed ID (PSIV) Dword.

With this mechanism may able to detect more kind of Protocol Speed
in USB3 and also compatiable with three kind of speed of USB2.

Cc: Jenny Huang <jenny.huang@intel.com>
Cc: More Shih <more.shih@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Ian Chiu <Ian.chiu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>

MdeModulePkg/Bus/Pci/XhciDxe/Xhci.h

@@ -227,6 +227,8 @@ struct _USB_XHCI_INSTANCE {
   UINT32                      ExtCapRegBase;
   UINT32                      UsbLegSupOffset;
   UINT32                      DebugCapSupOffset;
+  UINT32                      Usb2SupOffset;
+  UINT32                      Usb3SupOffset;
   UINT64                      *DCBAA;
   VOID                        *DCBAAMap;
   UINT32                      MaxSlotsEn;