usb: host: add dependence for USB_EHCI_MV

This ehci driver is mainly designed to support host controller found on
Marvell PXA and MMP Soc series.
Add the dependence to avoid the potential build failure which may
include two EHCI controller drivers.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>