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: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>