mtd: rawnand: omap: Add larger page NAND chips support

There is no reason to be limited to 4kiB page NAND chips just because
this is the maximum length the ELM is able to handle in one go. Just
call the ELM several times and it will process as many data as needed.

Here we introduce the concept of ECC page (which is at most 4kiB). The
ELM will be sought as many times as there are ECC pages.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Ryan Barnett <ryan.barnett@collins.com>
Link: https://lore.kernel.org/linux-mtd/20210610134906.3503303-6-miquel.raynal@bootlin.com