mtd: rawnand: brcmnand: fallback to detected ecc-strength, ecc-step-size

This change supports nand-ecc-step-size and nand-ecc-strength fields in
brcmnand DT node to be optional.
see: Documentation/devicetree/bindings/mtd/brcm,brcmnand.txt

If both nand-ecc-strength and nand-ecc-step-size are not specified in
device tree node for NAND, raw NAND layer does detect ECC information by
reading ONFI extended parameter page for parts using ONFI >= 2.1.
In case of non-ONFI NAND parts there could be a nand_id table entry with
ECC information. If there is valid device tree entry for nand-ecc-strength
and nand-ecc-step-size fields it still shall override the detected values.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>