gpio: mxs: implement get_direction callback

gpiolib's gpiod_get_direction() function returns the EINVAL error
if .get_direction callback is not defined.
The patch implements the callback for mxs chip which is useful
for debugging.

Inspired by arch/arm/mach-at91/gpio.c

On the moment the patch is required to get the patch
"serial: mxs-auart: enable PPS support" working.
It is planned to introduce new mctrl_gpio helpers to avoid
gpiod_get_direction() function.

Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>