ubi: Relax the 'no MLC' rule and allow MLCs operating in SLC mode

The MTD layer provides an SLC mode (purely software emulation of SLC
behavior) addressing the paired-pages corruption issue, which was the
main reason for refusing attaching MLC NANDs to UBI.

Relax this rule and allow partitions that have the
MTD_EMULATE_SLC_ON_MLC flag set to be attached.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Richard Weinberger <richard@nod.at>
Link: https://lore.kernel.org/linux-mtd/20200503155341.16712-9-miquel.raynal@bootlin.com