dt-bindings: mtd: brcmnand: Make nand-ecc-strength and nand-ecc-step-size optional

nand-ecc-strength and nand-ecc-step-size can be made optional as
brcmnand driver can support using raw NAND layer detected values.
Signed-off-by: NKamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>

Documentation/devicetree/bindings/mtd/brcm,brcmnand.txt

@@ -101,10 +101,10 @@ Required properties:
                               number (e.g., 0, 1, 2, etc.)
 - #address-cells            : see partition.txt
 - #size-cells               : see partition.txt
-- nand-ecc-strength         : see nand-controller.yaml
-- nand-ecc-step-size        : must be 512 or 1024. See nand-controller.yaml
 
 Optional properties:
+- nand-ecc-strength         : see nand-controller.yaml
+- nand-ecc-step-size        : must be 512 or 1024. See nand-controller.yaml
 - nand-on-flash-bbt         : boolean, to enable the on-flash BBT for this
                               chip-select. See nand-controller.yaml
 - brcm,nand-oob-sector-size : integer, to denote the spare area sector size