mtd: nand: Add a helper returning the number of eraseblocks per target

Some drivers in the raw NAND framework seems to need this helper, so
let's just add it instead of open-coding the logic.

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>