usb: add HAS_IOMEM dependency to USB_XHCI_HCD

drivers/built-in.o: In function `xhci_plat_probe':
/home/vegard/linux/drivers/usb/host/xhci-plat.c:160: undefined reference to `devm_ioremap_resource'

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>