gpio: add bindings for Technologic I2C-FPGA gpio controller

Device tree binding documentation for Technologic's I2C-FPGA GPIO
controller.
Signed-off-by: Lucile Quirion <lucile.quirion@savoirfairelinux.com>
[Quirked in a reference to gpio.txt]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-ts4900.txt

+* Technologic Systems I2C-FPGA's GPIO controller bindings
+
+This bindings describes the GPIO controller for Technologic's FPGA core.
+TS-4900's FPGA encodes the GPIO state on 3 bits, whereas the TS-7970's FPGA
+uses 2 bits: it doesn't use a dedicated input bit.
+
+Required properties:
+- compatible: Should be one of the following
+		"technologic,ts4900-gpio"
+		"technologic,ts7970-gpio"
+- reg: Physical base address of the controller and length
+       of memory mapped region.
+- #gpio-cells: Should be two. The first cell is the pin number.
+- gpio-controller: Marks the device node as a gpio controller.
+
+Optional property:
+- ngpios: Number of GPIOs this controller is instantiated with,
+  the default is 32. See gpio.txt for more details.
+
+Example:
+
+&i2c2 {
+	gpio8: gpio@28 {
+		compatible = "technologic,ts4900-gpio";
+		reg = <0x28>;
+		#gpio-cells = <2>;
+		gpio-controller;
+		ngpios = <32>;
+	};
+};