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>

include/linux/gpio/driver.h

@@ -22,6 +22,9 @@ enum gpio_lookup_flags;
 
 struct gpio_chip;
 
+#define GPIO_LINE_DIRECTION_IN	1
+#define GPIO_LINE_DIRECTION_OUT	0
+
 /**
  * struct gpio_irq_chip - GPIO interrupt controller
  */