usb: 64-bit architectures support for xHCI

This commit allows xHCI to use both 64 and 32 bit memory
physical addresses depending on architecture it's being built for.
Also it makes use of readq()/writeq() on 64-bit systems

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>