mtd: rawnand: mtk: Add validity check for CE# pin setting

Currently, we only check how many CE# pins are set in device tree.
But it should be necessary to check whether CE# pin setting is
duplicated or if CE# pin index exceeds the maximum CE# number that
controller supports.

So, add validity check to avoid these invalid settings.

Signed-off-by: Xiaolei Li <xiaolei.li@mediatek.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>