usb: xhci: plat: add ->plat_start() and ->init_quirk() methods

these two methods will be used to hide
platform-specific details so we can get rid of
xhci_plat_type_is() in a later patch.

Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>