of: overlay: rename variables to be consistent

Variables change name across function calls when there is not a good
reason to do so.  Fix by changing "fdt" to "new_fdt" and "tree" to
"overlay_root".

The name disparity was confusing when creating the following commit.
The name changes are in this separate commit to make review of the
following commmit less complex.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220420222505.928492-2-frowand.list@gmail.com