MAINTAINERS: Add an entry for omap related .dts files

All your omap .dts files are belong to us.

Benoît has been doing a good job picking up most of the
omap .dts files so far. Let's make sure we both get
cc:ed for the related patches.

The .dts patches need to be queued by us as separate
patches from drivers and other code changes to avoid
pointless merge conflists like we saw with v3.8
networking changes.

Cc: Benoît Cousson <b-cousson@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>