mtd: physmap_of: assign parent for the concatenated MTD

If there is more than one map region for this device, then the
concatenated MTD will not have a parent device assigned to it -- only
the sub-devices (which are not actually registered with the framework)
will have their parents assigned. Let's assign the concatenated device
correctly.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>