cmd: mtdparts: accept spi-nand devices

Let spi-nand devices be recognized by mtdparts. This is superfluous
but a full mtdparts rework would be very time-consuming.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Stefan Roese <sr@denx.de>