ARM: dts: Add board dts file for Snow board (ARM Chromebook)

ARM: dts: Add board dts file for Snow board (ARM Chromebook)

This dts file is based on the Snow dts file in the Chromium OS kernel
tree with the following changes:
* Some details have been updated to match the exynos5250-smdk5250.dts
  file from linux-next (as of c11068538994430547722dc9fb515a0ceefd5cb9).
* This file doesn't include references to hardware whose upstream
  support isn't quite there yet.  That includes most i2c devices.
  Note that most i2c busses have been included with no devices.

The Snow dts file is mostly just an include of the "cros5250" dts file
which describes a class of similar boards. Support for other boards has
not yet been send upstream.

With this file and a change to use UART3 for serial output I can:
* Boot to a command line using either SD or EMMC as a root filesystem
* See the power button and lid switch using evtest.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>