Pass an optional device_node pointer in the platform data, which in turn
will be put into a mtd_part_parser_data. This way, code that sets up the
platform devices can pass along the node from DT so that the partitions
can be parsed.

For non-DT boards, this change has no effect.
Signed-off-by: NDaniel Mack <zonque@gmail.com>
Acked-by: NGrant Likely <grant.likely@secretlab.ca>
Acked-by: NArtem Bityutskiy <dedekind1@gmail.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>