usb: xhci: cleanup xhci_hcd allocation

HCD core allocates memory for HCD private data in
usb_create_[shared_]hcd() so make use of that
mechanism to allocate the struct xhci_hcd.

Introduce struct xhci_driver_overrides to provide
the size of HCD private data and hc_driver operation
overrides. As of now we only need to override the
reset and start methods.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>