of: overlay: check prevents multiple fragments add or delete same node

Multiple overlay fragments adding or deleting the same node is not
supported.  Replace code comment of such, with check to detect the
attempt and fail the overlay apply.

Devicetree unittest where multiple fragments added the same node was
added in the previous patch in the series.  After applying this patch
the unittest messages will no longer include:

   Duplicate name in motor-1, renamed to "controller#1"
   OF: overlay: of_overlay_apply() err=0
   ### dt-test ### of_overlay_fdt_apply() expected -22, ret=0, overlay_bad_add_dup_node
   ### dt-test ### FAIL of_unittest_overlay_high_level():2419 Adding overlay 'overlay_bad_add_dup_node' failed

   ...

   ### dt-test ### end of unittest - 210 passed, 1 failed

but will instead include:

   OF: overlay: ERROR: multiple overlay fragments add and/or delete node /testcase-data-2/substation@100/motor-1/controller

   ...

   ### dt-test ### end of unittest - 211 passed, 0 failed

Tested-by: Alan Tull <atull@kernel.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>