gpio/tegra: dt: add binding for gpio polarity

Allocate one bit in the available extra cell to indicate if the gpio
should be considered logically inverted.
Signed-off-by: NOlof Johansson <olof@lixom.net>
Acked-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/gpio/gpio_nvidia.txt

@@ -3,5 +3,6 @@ NVIDIA Tegra 2 GPIO controller
 Required properties:
 - compatible : "nvidia,tegra20-gpio"
 - #gpio-cells : Should be two. The first cell is the pin number and the
-  second cell is used to specify optional parameters (currently unused).
+  second cell is used to specify optional parameters:
+  - bit 0 specifies polarity (0 for normal, 1 for inverted)
 - gpio-controller : Marks the device node as a GPIO controller.