usb: host: xhci-plat: add struct xhci_plat_priv

This patch adds struct xhci_plat_priv to simplify the code to match
platform specific variables. For now, this patch adds a member "type"
in the structure.

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>