uboot-imx

board: toradex: use get_nand_dev_by_index()

As part of preparation for nand DM conversion the new API has been
introduced to remove direct access to nand_info array. So, use it here
instead of accessing to nand_info array directly.

Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>