We plan to move cache related fields to a pagecache struct in nand_chip
but some drivers access ->pagebuf directly to invalidate the cache
before they start using ->data_buf.

Let's provide an helper that returns a pointer to ->data_buf after
invalidating the cache.
Signed-off-by: NBoris Brezillon <bbrezillon@kernel.org>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: NFrieder Schrempf <frieder.schrempf@kontron.de>