mtd: rawnand: Move all page cache related fields to a sub-struct

Looking at the field names it's hard to tell what ->data_buf, ->pagebuf
and ->pagebuf_bitflips are for. Clarify that by moving those fields
in a sub-struct named pagecache.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>