Add binding documentation for CRIS

Axis Communications AB

ARTPEC series SoC Device Tree Bindings

CRISv32 based SoCs are ETRAX FS and ARTPEC-3:

    - compatible = "axis,crisv32";

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:

* 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>;

	};