mtd: cmdlinepart: Add an slc option to use SLC mode on a part

Add a new option to set the MTD_SLC_ON_MLC_EMULATION flag.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200503155341.16712-8-miquel.raynal@bootlin.com