mtd: atmel_nand: make pmecc-cap, pmecc-sector-size in dts is optional.

If those two are not specified in dts file, driver will report an error.

TODO: in this case, driver will find ecc requirement in NAND ONFI parameters.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>