dt-bindings: mtd: partition: Document the slc-mode property

Add a boolean property to force a specific partition attached to an MLC
NAND to be accessed in an emulated SLC mode this making this partition
immune to paired-pages corruptions.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200503155341.16712-6-miquel.raynal@bootlin.com