mtd: rawnand: gpmi: add support for specific ECC strength

Add support for specified ECC strength/size using device tree
properties nand-ecc-strength/nand-ecc-step-size.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>