Enable OF_UPSTREAM to use upstream DT and add a ti/omap/ prefix to the
DEFAULT_DEVICE_TREE config option.

That way, a DTS from the upstream dts/upstream/src/ directory is used
instead of the arch/$(ARCH)/dts/ directory. These in turn are removed.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Enric Balletbo i Serra <eballetbo@gmail.com>