devicetree: Add ARM pl061 gpio controller binding doc

Add binding documentation for ARM's Primecell PL061 GPIO controller.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

Documentation/devicetree/bindings/gpio/pl061-gpio.txt

+ARM PL061 GPIO controller
+
+Required properties:
+- compatible : "arm,pl061", "arm,primecell"
+- #gpio-cells : Should be two. The first cell is the pin number and the
+  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.
+- interrupts : Interrupt mapping for GPIO IRQ.
+