mtd: partitions: ofpart: Parse the slc-mode property

Parse the slc-mode property and set the MTD_MLC_IN_SLC_MODE flag
when present.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200503155341.16712-7-miquel.raynal@bootlin.com