mtd: nand: atmel: Fix DT backward compatibility in pmecc.c

PMECC caps extraction from old DT bindings is broken, thus leading to
erroneous EL registers offset, which in turn make HW ECC unusable on
sama5d2 when old bindings are in use.

Passing the NAND dev node instead of the NFC node to of_match_node()
solves the problem.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: f88fc122 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Cc: <stable@vger.kernel.org>
Tested-by: Romain Izard <romain.izard.pro@gmail.com>