arm/dts: Add minimal device tree support for omap2420 and omap2430

Add minimal device tree support for omap2420 and omap2430.

This is needed to keep the uart functional on omap2 after
omap_serial_init is removed from board-generic.c.

Reviewed-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>