mtd: mtdpart: Support MTDPART_SIZ_FULL in fixed-partitions

mtd: mtdpart: Support MTDPART_SIZ_FULL in fixed-partitions

Flash partitions may specify MTDPART_SIZ_FULL (=0) as the size of the
partition to indicate "the remainder of the flash". Make this work with
device-tree "fixed-partitions" as well.

This makes MTD partitioning compatible with the Linux kernel, see:
  https://github.com/torvalds/linux/blob/master/include/linux/mtd/partitions.h#L29
  https://github.com/torvalds/linux/blob/master/drivers/mtd/mtdpart.c#L123

Previously, this could only be done through MTDPARTS so this change allows
boards like topic_miami to migrate from `mtdparts`/`mtdids` to devicetree
partitions.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>