mtd: parsers: ar7: remove support

AR7 is going to be removed from the Kernel, so remove its support for
MTD.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>