mtd: rawnand: Add NV-DDR timings

Create the relevant ONFI NV-DDR timings structure and fill it with
default values from the ONFI specification.

Add the relevant structure entries and helpers.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-9-miquel.raynal@bootlin.com