dt-bindings: mtd: add binding for Linksys Northstar partitions

Linksys on Broadcom Northstar devices uses fixed flash layout with
multiple firmware partitions.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210312134919.7767-1-zajec5@gmail.com