usb: ehci: Allow EHCI to be built on OMAP3

OMAP3 chips have a built-in EHCI controller.
The recently introduced omap ehci-hcd driver missed
out on selecting USB_ARCH_HAS_EHCI in Kconfig.

Without this, the driver cannot be built.
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Cc: Vikram Pandita <vikram.pandita@ti.com>
Cc: Ajay Kumar Gupta <ajay.gupta@ti.com>
Acked-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>