mtd: ofpart: move ofpart partitions to a dedicated dt node

Parsing direct subnodes of a mtd device as partitions is unreliable
since the mtd device is also part of its bus subsystem and can contain
bus data in subnodes.

Move ofpart data to a subnode of its own so it is clear which data is
part of the partition layout.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>