mtd: partitions: Add OF support to RedBoot partitions

This adds device tree support for RedBoot partitioning. We
read out the FIS directory block information from the device
tree and then parse the partition table from there.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>