doc: dt: mtd: support partitions in a special 'partitions' subnode

To avoid conflict with other drivers using subnodes of the mtd device
create only one ofpart-specific node rather than any number of
arbitrary partition subnodes.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>