mtd: Add support for emulated SLC mode on MLC NANDs

MLC NANDs can be made a bit more reliable if we only program the lower
page of each pair. At least, this solves the paired-pages corruption
issue.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200503155341.16712-5-miquel.raynal@bootlin.com