mtd: mtdpart: implement proper partition handling

mtd: mtdpart: implement proper partition handling

Instead of collecting partitions in a flat list, create a hierarchy
within the mtd_info structure: use a partitions list to keep track of
the partitions of an MTD device (which might be itself a partition of
another MTD device), a pointer to the parent device (NULL when the MTD
device is the root one, not a partition).

By also saving directly in mtd_info the offset of the partition, we
can get rid of the mtd_part structure.

Signed-off-by: Miquel Raynal <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Reviewed-by: Boris Brezillon <[email protected]>