imx6: aristainetos: fix NAND detection with latest mainline

commit 88718be3 ("mtd: rename CONFIG_NAND -> CONFIG_MTD_RAW_NAND")
moved CONFIG_NAND -> CONFIG_MTD_RAW_NAND. Adapt board code to this
change, as last merge did not respect the above commit.

Signed-off-by: Heiko Schocher <hs@denx.de>