gpio: omap: implement get_direction

This patch implements gpio_chip's get_direction() routine, that
lets other drivers get particular GPIOs direction using
struct gpio_desc.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>