imx6ulevk: fix 74LV OE usage

imx6ulevk: fix 74LV OE usage

Fix 74LV OE gpio index. gpio index is wrong,
so gpio output will not have effect, since we
use wrong GPIO5_IO18, but not correct GPIO5_IO8.

And at the end of the initialization of 74lv init, should
keep OE voltage level at LOW to make 74lv output the correct
voltage.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>