gpio: add DT bindings for Cortina Gemini GPIO

This adds fairly standard DT bindings for the Cortina Systems
Gemini GPIO controller.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/cortina,gemini-gpio.txt

+Cortina Systems Gemini GPIO Controller
+
+Required properties:
+
+- compatible : Must be "cortina,gemini-gpio"
+- reg : Should contain registers location and length
+- interrupts : Should contain the interrupt line for the GPIO block
+- gpio-controller : marks this as a GPIO controller
+- #gpio-cells : Should be 2, see gpio/gpio.txt
+- interrupt-controller : marks this as an interrupt controller
+- #interrupt-cells : a standard two-cell interrupt flag, see
+  interrupt-controller/interrupts.txt
+
+Example:
+
+gpio@4d000000 {
+	compatible = "cortina,gemini-gpio";
+	reg = <0x4d000000 0x100>;
+	interrupts = <22 IRQ_TYPE_LEVEL_HIGH>;
+	gpio-controller;
+	#gpio-cells = <2>;
+	interrupt-controller;
+	#interrupt-cells = <2>;
+};