ARM: tegra: colibri_t20: fix nand pinmux

Pingroup ATC seems to come out of reset with config set to NAND, so we
need to explicitly configure some other function to this group in order
to avoid clashing settings.

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>