gpio: Add definition for GPIO direction

At least for me it is difficult to remember the meaning of GPIO
direction values. Define GPIO_LINE_DIRECTION_IN and
GPIO_LINE_DIRECTION_OUT so that occasional GPIO contributors would
not need to always check the meaning of hard coded values 1 and 0.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>