libfdt: overlay_merge: Copy over various nodes and their properties

Many overlay fragment nodes that don't have a target node to be overlaid in base
device-tree will need to be copied over as-is into base-tree. This includes
merging properties found in special nodes such as __fixups__, __symbols__ and
__local_fixups__

Change-Id: I15c7e0432bda77837bd575d235536d8191dd1820
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>