mtd: parsers: trx: Allow to specify brcm, trx-magic in DT

Buffalo uses a different TRX magic for every device, to be able to use
this trx parser, make it possible to specify the TRX magic in device
tree. If no TRX magic is specified in device tree, the standard value
will be used. This value should only be specified if a vendor chooses to
use a non standard TRX magic.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210418214616.239574-3-hauke@hauke-m.de