dt: add helper for empty dt creation

dt: add helper for empty dt creation

We want to get rid of the concept of loading an external device tree and instead
generate our own. However, to do this we need to also create a device tree
template programatically.

This patch adds a helper to create an empty device tree in memory.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@petalogix.com>