dt-bindings: mtd: brcm,trx: Add brcm,trx-magic

This adds the description of an additional property which allows to
specify a custom partition parser magic to detect a trx partition.
Buffalo has multiple device which are using the trx format, but with
different magic values.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210418214616.239574-2-hauke@hauke-m.de