MAINTAINERS: Add the IRC channel to the MTD related subsystems

The #mtd channel (on OFTC servers) is being used to discuss MTD related
topics. Add it for better visibility to the HYPERBUS, NAND and SPI NOR
entries.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>