dtoc: Change dst to self in debug message

Fix the error message to not use dst and use self as it is copying the
properties to self.

While using templating if there are no subnodes defined, we end up in
this situation where "dst" isn't defined and it tries to print the error
message and fails.

'UnboundLocalError: local variable 'dst' referenced before assignment'

Fixes: 55e1278d ("dtoc: Allow inserting a list of nodes into another")

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>