usb: host: xhci-plat: add xhci_plat_start()

Some platforms (such as the Renesas R-Car) need to initialize some specific
registers after xhci driver calls usb_add_hcd() and before the driver calls
xhci_run(). So, this patch adds the xhci_plat_start() function.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>