dt-bindings: qcom_nandc: update for ECC strength and step size

1. If nand-ecc-strength specified in DT, then controller will use
   this ECC strength otherwise ECC strength will be calculated
   according to chip requirement and available OOB size.

2. QCOM NAND controller supports only one step size (512 bytes) but
   nand-ecc-step-size is required property in DT. This DT property
   can be removed and ecc step size can be assigned in driver with
   512 bytes value.

Signed-off-by: Abhishek Sahu <absahu@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>