usb: Add Device Tree support to XHCI Platform driver

Add Device Tree match table to xhci-plat.c. Add DT bindings document.

Signed-off-by: Al Cooper <alcooperx@gmail.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>