of: overlay: simplify applying symbols from an overlay

The code to apply symbols from an overlay to the live device tree
was implemented with the intent to be minimally intrusive on the
existing code.  After recent restructuring of the overlay apply
code, it is easier to disintangle the code that applies the
symbols, and to make the overlay changeset creation code more
straight forward and understandable.

Remove the extra complexity, and make the code more obvious.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>