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>

Documentation/devicetree/bindings/mtd/gpmi-nand.txt

@@ -47,6 +47,11 @@ Optional properties:
                        partitions written from Linux with this feature
                        turned on may not be accessible by the BootROM
                        code.
+  - nand-ecc-strength: integer representing the number of bits to correct
+                       per ECC step. Needs to be a multiple of 2.
+  - nand-ecc-step-size: integer representing the number of data bytes
+                       that are covered by a single ECC step. The driver
+                       supports 512 and 1024.
 
 The device tree may optionally contain sub-nodes describing partitions of the
 address space. See partition.txt for more detail.