ARM: tegra: Add Colibri T20 512MB COM device tree

This adds the device tree include file for the Toradex Colibri T20
Computer on Module (COM). It's only valid for the 512MB RAM version of
the module, as the 256MB version needs different EMC tables and flash
configuration. To make this clear the suffix -512 was added to the board
compatible string.

The Colibri T20 uses a Tegra20 SoC and has onboard USB Ethernet and AC97
sound.

Still some things like onboard NAND support missing, but should be a
good base for further development.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>