gpio: generic: add get_direction support

Allow to determine the current direction configuration by
reading back from the direction register.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>