dt-bindings: mfd: Add Altera Arria10 System Resource Chip bindings

The Altera Arria10 Devkit System Resource chip is a Multi-Function
Device with a GPIO expander.

This patch adds documentation for the Altera A10-SR DT bindings.
Signed-off-by: NThor Thayer <tthayer@opensource.altera.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/altera-a10sr.txt

+* Altera Arria10 Development Kit System Resource Chip
+
+Required parent device properties:
+- compatible		: "altr,a10sr"
+- spi-max-frequency	: Maximum SPI frequency.
+- reg			: The SPI Chip Select address for the Arria10
+			  System Resource chip
+- interrupt-parent	: The parent interrupt controller.
+- interrupts		: The interrupt line the device is connected to.
+- interrupt-controller	: Marks the device node as an interrupt controller.
+- #interrupt-cells	: The number of cells to describe an IRQ, should be 2.
+			    The first cell is the IRQ number.
+			    The second cell is the flags, encoded as trigger
+			    masks from ../interrupt-controller/interrupts.txt.
+
+The A10SR consists of these sub-devices:
+
+Device                   Description
+------                   ----------
+a10sr_gpio               GPIO Controller
+
+Arria10 GPIO
+Required Properties:
+- compatible        : Should be "altr,a10sr-gpio"
+- gpio-controller   : Marks the device node as a GPIO Controller.
+- #gpio-cells       : Should be two.  The first cell is the pin number and
+                      the second cell is used to specify flags.
+                      See ../gpio/gpio.txt for more information.
+
+Example:
+
+        resource-manager@0 {
+		compatible = "altr,a10sr";
+		reg = <0>;
+		spi-max-frequency = <100000>;
+		interrupt-parent = <&portb>;
+		interrupts = <5 IRQ_TYPE_LEVEL_LOW>;
+		interrupt-controller;
+		#interrupt-cells = <2>;
+
+		a10sr_gpio: gpio-controller {
+			compatible = "altr,a10sr-gpio";
+			gpio-controller;
+			#gpio-cells = <2>;
+		};
+	};