dt-bindings: mtd: rawnand: gpmi: document specific ECC strength

Document newly supported device tree properties nand-ecc-strength/
nand-ecc-step-size to specify ECC strength/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>