USB: select USB_ARCH_HAS_EHCI for MXS

Commit 09f6ffde (USB: EHCI: fix build error by making ChipIdea host a
normal EHCI driver) introduced a dependency on USB_EHCI_HCD for the
chipidea USB host driver, that in turns depends on USB_ARCH_HAS_EHCI.

If this symbol is not set for MXS, the MXS boards are not able to use
the chipidea driver anymore.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>