dt-bindings: mtd: partitions: convert BCM47xx to the json-schema

This helps validating DTS files.

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/20220216104135.31307-1-zajec5@gmail.com