ARM: sun7i/sun6i: irqchip: Update the documentation

Add documentation for NMI irqchip.
Signed-off-by: Carlo Caione <carlo@caione.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-sunxi@googlegroups.com
Cc: mark.rutland@arm.com
Cc: hdegoede@redhat.com
Acked-by: maxime.ripard@free-electrons.com
Link: http://lkml.kernel.org/r/1395256879-8475-4-git-send-email-carlo@caione.orgSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/devicetree/bindings/interrupt-controller/allwinner,sun67i-sc-nmi.txt

+Allwinner Sunxi NMI Controller
+==============================
+
+Required properties:
+
+- compatible : should be "allwinner,sun7i-a20-sc-nmi" or
+  "allwinner,sun6i-a31-sc-nmi"
+- reg : Specifies base physical address and size of the registers.
+- interrupt-controller : Identifies the node as an interrupt controller
+- #interrupt-cells : Specifies the number of cells needed to encode an
+  interrupt source. The value shall be 2. The first cell is the IRQ number, the
+  second cell the trigger type as defined in interrupt.txt in this directory.
+- interrupt-parent: Specifies the parent interrupt controller.
+- interrupts: Specifies the interrupt line (NMI) which is handled by
+  the interrupt controller in the parent controller's notation. This value
+  shall be the NMI.
+
+Example:
+
+sc-nmi-intc@01c00030 {
+	compatible = "allwinner,sun7i-a20-sc-nmi";
+	interrupt-controller;
+	#interrupt-cells = <2>;
+	reg = <0x01c00030 0x0c>;
+	interrupt-parent = <&gic>;
+	interrupts = <0 0 4>;
+};