arm/dts: Add omap36xx.dtsi file and rename omap3-beagle to omap3-beagle-xm

The extra serial port is not available on 34xx. And the current
omap3-beagle.dts file is for omap3-beagle-xm.dts as it lists 512MB
of memory.

Please somebody submit a new omap3-beagle.dts for the original 34xx
BeagleBoard after testing it properly.

Cc: Benoit Cousson <b-cousson@ti.com>
Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Tony Lindgren <tony@atomide.com>