OMAP: OMAPFB: add omapdss device

The upcoming new display subsystem driver is divided to two devices,
omapdss and omapfb, of which omapdss handles the actual hardware.

This patch adds a dummy omapdss platform device for the current omapfb
driver, which is then used to get the clocks. This will make it possible
for the current and the new display drivers to co-exist.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Acked-by: Tony Lindgren <tony@atomide.com>