mtd: nand: atmel: rely on generic DT parsing done in nand_scan_ident()

The core now takes care of parsing generic DT properties in
nand_scan_ident() when nand_set_flash_node() has been called.
Rely on this initialization instead of calling of_get_nand_xxx()
manually.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@atmel.com>