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.
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>