Add gpio_set_direction support. This makes the driver support
GPIO input/output dynamically change from userspace.

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Fugang Duan <fugang.duan@nxp.com>
Cc: Bai Ping <ping.bai@nxp.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>