Documentation: dt: OMAP: Update Overo/Tobi

Update the compatible string for Overo/Tobi to reflect the latest
changes.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>