ARM: dts: Configure earlycon for pandaboard

With commit 8dd6666f ("ARM: OMAP2+: omap_hwmod: Add support for
earlycon") we can now get debug information early if something goes
wrong as long as kernel command line has earlycon in it. Let's enable
it for pandaboards as they all have a debug uart at uart3.

Signed-off-by: Tony Lindgren <tony@atomide.com>