dt-bindings: mtd: document Broadcom's BCM47xx partitions

Broadcom based home router devices use partitions which have to be
discovered in a specific way. They are not fixed and there is not any
standard partition table. This commit adds and describes a new custom
binding for such devices.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>