mtd: nand: timings: Fix tADL_min for ONFI 4.0 chips

ONFI 4.0 spec defines different values for the tADL_min timing.
Since we don't want to have different timings depending on the ONFI
version, we just set tADL_min to the maximum value (the one specified
in the ONFI 4.0 spec).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>