dt: add helper for phandle references

dt: add helper for phandle references

Phandles are the fancy device tree name for "pointer to another node".
To create a phandle property, we most likely want to reference to the
node we're pointing to by its path. So create a helper that allows
us to do so.

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