ARM: tegra: Add Toradex Iris carrier board DT with T20 512MB COM

This adds the device tree for the Toradex Iris carrier board used
together with a Colibri T20 512MB COM.

The Iris has the following features, in brackets the current status:
- DVI and VGA output through DVI-I connector (DVI-D enabled and tested)
- LVDS output
- 1 USB host port (enabled and tested)
- 1 USB OTG port (enabled)
- 100 MBit Ethernet (enabled and tested)
- 5 UART ports  (2 on 10way headers enabled and tested)
- 1 MicroSD Slot (enabled and tested)
- Audio connectors (enabled, only HP out and Line-in tested)
- i2c RTC
- GPIO connector (enabled, only sparsely tested)
- external i2c bus
- 4 PWM out
- analog in

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