dt-bindings: nand: meson: support for 512B ECC step size

Meson NAND supports both 512B and 1024B ECC step size, so replace
'const' for only 1024B step size with enum for both sizes.

Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230711122129.2635558-2-AVKrasnov@sberdevices.ru