ARM: dts: omap3-tobi: Use the correct vendor prefix

Gumstix is the correct vendor for all Overo related products.

Reported-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>