mtd: nand: Introduce a block iterator

In order to be able to iterate easily across eraseblocks rather than
pages, let's introduce a block iterator inspired from the page iterator.

The main usage of this iterator will be for continuous/sequential reads,
where it is interesting to use a single request rather than split the
requests in smaller chunks (ie. pages) that can be hardly optimized.

So a "continuous" boolean get's added for this purpose.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240826101412.20644-3-miquel.raynal@bootlin.com