ARM: dts: omap: add channel to DSI panels

The standard binding for DSI requires that the channel number
of the panel is encoded in DT. This adds the channel number in
all OMAP3-5 boards in preparation for using common infrastructure.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>