cmd: mtd: Use the subcmd infrastructure to declare mtd sub-commands

It's way simpler this way, and we also gain auto-completion support for
free (MTD name auto-completion has been added with mtd_name_complete())

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>