ARM: dts: Configure earlycon for omap5-common

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 omap5-common as all these have debug uart at uart3.

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