ARM: dts: OMAP3: Add support for Gumstix Overo with Tobi expansion board

The Gumstix Overo is a computer on module using an OMAP3 processor.
This module must be plugged into an expansion board.

This patch adds a first device tree support for the Overo, using the
Tobi expansion board. The current support is able to boot and mount
the rootfs from MMC.

This patche also updates the omap3 dtb build target.

Currently working:
- mmc0 (on board microSD)
- i2c0 and i2c2 (i2c1 not used)
- led on GPIO

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>