arm: omap4: select USB_ARCH_HAS_EHCI

The OMAP4 has an on-chip EHCI controller. Select USB_ARCH_HAS_EHCI
to allow the EHCI driver to be built on OMAP4.

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>