mtd: uclass: add probe function

The user might want to trigger the probe of any MTD device, export these
functions so they can be called from a command source file.

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