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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

Documentation/devicetree/bindings/mtd/partitions/brcm,trx.txt

···
 Required properties:
 - compatible : (required) must be "brcm,trx"
 
+Optional properties:
+
+- brcm,trx-magic: TRX magic, if it is different from the default magic
+		  0x30524448 as a u32.
+
 Example:
 
 flash@0 {