Add binding documentation for CRIS

Only includes the devboard 88 (CRISv32) at the moment.
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>

Documentation/devicetree/bindings/cris/axis.txt

+Axis Communications AB
+ARTPEC series SoC Device Tree Bindings
+
+
+CRISv32 based SoCs are ETRAX FS and ARTPEC-3:
+
+    - compatible = "axis,crisv32";
+
+

Documentation/devicetree/bindings/cris/boards.txt

+Boards based on the CRIS SoCs:
+
+Required root node properties:
+    - compatible = should be one or more of the following:
+	- "axis,dev88"	- for Axis devboard 88 with ETRAX FS
+
+Optional:
+

Documentation/devicetree/bindings/cris/interrupts.txt

+* CRISv32 Interrupt Controller
+
+Interrupt controller for the CRISv32 SoCs.
+
+Main node required properties:
+
+- compatible : should be:
+	"axis,crisv32-intc"
+- interrupt-controller : Identifies the node as an interrupt controller
+- #interrupt-cells : Specifies the number of cells needed to encode an
+  interrupt source. The type shall be a <u32> and the value shall be 1.
+- reg: physical base address and size of the intc registers map.
+
+Example:
+
+	intc: interrupt-controller {
+		compatible = "axis,crisv32-intc";
+		reg = <0xb001c000 0x1000>;
+		interrupt-controller;
+		#interrupt-cells = <1>;
+	};
+
+