usb: host: xhci: add a platform-private field

This patch adds an xhci->priv field for private use by
XHCI platform drivers. Until now none of the platform drivers
has used this private space.

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