mtd: nand: add support for "nand-ecc-algo" DT property

So far it was only possible to specify ECC algorithm using "soft" and
"soft_bch" values of nand-ecc-mode prop. There wasn't a way to specify
it for a hardware ECC mode.

Now that we have independent field in NAND subsystem for storing info
about ECC algorithm we may also add support for this new DT property.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>