ARM: add flat device tree support

Based on other architectures already supported.
Tested on OMAP3 Beagle board and another unnamed ARM platform.

Signed-off-by: John Rigby <john.rigby@linaro.org>
Tested-by: Rob Herring <rob.herring@smooth-stone.com>
Acked-by: Wolfgang Denk <wd@denx.de>