gpio: sa1100: implement get_direction method

Allow gpiolib to read back the current IO direction configuration by
implementing the .get_direction callback.  This, in part, allows
debugfs to report the complete true hardware state rather than the
software state.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>