dt-bindings: Document the Synopsys GPIO via CREG bindings

This patch adds documentation of device tree bindings for the Synopsys
GPIO via CREG driver.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/snps,creg-gpio.txt

+Synopsys GPIO via CREG (Control REGisters) driver
+
+Required properties:
+- compatible : "snps,creg-gpio-hsdk" or "snps,creg-gpio-axs10x".
+- reg : Exactly one register range with length 0x4.
+- #gpio-cells : Since the generic GPIO binding is used, the
+  amount of cells must be specified as 2. The first cell is the
+  pin number, the second cell is used to specify optional parameters:
+  See "gpio-specifier" in .../devicetree/bindings/gpio/gpio.txt.
+- gpio-controller : Marks the device node as a GPIO controller.
+- ngpios: Number of GPIO pins.
+
+Example:
+
+gpio: gpio@f00014b0 {
+	compatible = "snps,creg-gpio-hsdk";
+	reg = <0xf00014b0 0x4>;
+	gpio-controller;
+	#gpio-cells = <2>;
+	ngpios = <2>;
+};