omap3: Allow EHCI to be built on OMAP3

OMAP34XX has EHCI, so select USB_ARCH_HAS_EHCI.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>