mtd: parsers: trx: allow to use on MediaTek MIPS SoCs

Buffalo sells some router devices which have trx-formatted firmware,
based on MediaTek MIPS SoCs. To use parser_trx on those devices, add
"RALINK" to dependency and allow to compile for MediaTek MIPS SoCs.

examples:

- WCR-1166DS  (MT7628)
- WSR-1166DHP (MT7621)
- WSR-2533DHP (MT7621)

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220213064045.1781-1-musashino.open@gmail.com