ARM: at91: gpio: implement get_direction

This is needed for gpiod_get_direction().
Otherwise, it returns -EINVAL.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>