DM644x platform and board setup

This adds platform and board setup changes required to support
vpfe capture driver on DM644x

Tested video capture on DM6446 with tvp514x driver
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Reviewed-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Muralidharan Karicheri <m-karicheri2@ti.com>
Signed-off-by: Denys Dmytriyenko <denis@denix.org>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>