libfdt: Add subnode at end of parent

When merging two overlays, the order of the fragments matters because
it preserves the order in which the fragments should be applied on the
final devicetree. Thus, subnodes should always be added as last child to
the parent, not first.

Change-Id: Iaff4b8ac0996222318d1e2b8590117249565755b
Signed-off-by: Elliot Berman <eberman@codeaurora.org>