of: overlay: test case of two fragments adding same node

Multiple overlay fragments adding or deleting the same node is not
supported.  An attempt to do so results in an incorrect devicetree.
The node name will be munged for the second add.

After adding this patch, the unittest messages will show:

   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

The incorrect (munged) node name "controller#1" can be seen in the
/proc filesystem:

   $ pwd
   /proc/device-tree/testcase-data-2/substation@100/motor-1
   $ ls
   compatible    controller    controller#1  name          phandle       spin
   $ ls controller
   power_bus
   $ ls controller#1
   power_bus_emergency

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