ARM: dt: add header to define GPIO flags

Many GPIO device tree bindings use the same flags. Create a header to
define those.
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Acked-by: NRob Herring <rob.herring@calxeda.com>

include/dt-bindings/gpio/gpio.h

+/*
+ * This header provides constants for most GPIO bindings.
+ *
+ * Most GPIO bindings include a flags cell as part of the GPIO specifier.
+ * In most cases, the format of the flags cell uses the standard values
+ * defined in this header.
+ */
+
+#ifndef _DT_BINDINGS_GPIO_GPIO_H
+#define _DT_BINDINGS_GPIO_GPIO_H
+
+#define GPIO_ACTIVE_HIGH 0
+#define GPIO_ACTIVE_LOW 1
+
+#endif