gpio: Add Netlogic XLP GPIO devicetree bindings documentation

Provide documentation for Netlogic XLP SoCs GPIO controller DTS bindings.
Signed-off-by: NKamlakant Patel <kamlakant.patel@broadcom.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

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

+Netlogic XLP Family GPIO
+========================
+
+This GPIO driver is used for following Netlogic XLP SoCs:
+	XLP832, XLP316, XLP208, XLP980, XLP532
+
+Required properties:
+-------------------
+
+- compatible: Should be one of the following:
+  - "netlogic,xlp832-gpio": For Netlogic XLP832
+  - "netlogic,xlp316-gpio": For Netlogic XLP316
+  - "netlogic,xlp208-gpio": For Netlogic XLP208
+  - "netlogic,xlp980-gpio": For Netlogic XLP980
+  - "netlogic,xlp532-gpio": For Netlogic XLP532
+- reg: Physical base address and length of the controller's registers.
+- #gpio-cells: Should be two. The first cell is the pin number and the second
+  cell is used to specify optional parameters (currently unused).
+- gpio-controller: Marks the device node as a GPIO controller.
+- nr-gpios: Number of GPIO pins supported by the controller.
+- interrupt-cells: Should be two. The first cell is the GPIO Number. The
+  second cell is used to specify flags. The following subset of flags is
+  supported:
+  - trigger type:
+	1 = low to high edge triggered.
+	2 = high to low edge triggered.
+	4 = active high level-sensitive.
+	8 = active low level-sensitive.
+- interrupts: Interrupt number for this device.
+- interrupt-parent: phandle of the parent interrupt controller.
+- interrupt-controller: Identifies the node as an interrupt controller.
+
+Example:
+
+	gpio: xlp_gpio@34000 {
+		compatible = "netlogic,xlp316-gpio";
+		reg = <0 0x34100 0x1000
+		       0 0x35100 0x1000>;
+		#gpio-cells = <2>;
+		gpio-controller;
+		nr-gpios = <57>;
+
+		#interrupt-cells = <2>;
+		interrupt-parent = <&pic>;
+		interrupts = <39>;
+		interrupt-controller;
+	};