ARM: mxs_defconfig: Make USB host functional again

commit 09f6ffde (USB: EHCI: fix build error by making ChipIdea host a normal
EHCI driver) introduced CONFIG_USB_EHCI_HCD as a dependency for USB_CHIPIDEA_HOST.

Select CONFIG_USB_EHCI_HCD, so that USB host can be functional again.

Cc: <stable@vger.kernel.org> # 3.8
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>