gpio: 74x164: make oe-pins optional

Make oe-pins optional because some boards have fixed it to enable.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>