mtd: rawnand: Use more recent ONFI specification wording

In particular, first ONFI specifications referred to SDR modes as
asynchronous modes, which is not the term we usually have in mind. The
spec has then been updated, so do the same here in the NAND subsystem to
avoid any possible confusion.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-7-miquel.raynal@bootlin.com