dt-bindings: Add DT bindings document for Broadcom SBA RAID driver

* Broadcom SBA RAID engine

Required properties:

- compatible: Should be one of the following

	      "brcm,iproc-sba"

	      "brcm,iproc-sba-v2"

  The "brcm,iproc-sba" has support for only 6 PQ coefficients

  The "brcm,iproc-sba-v2" has support for only 30 PQ coefficients

- mboxes: List of phandle and mailbox channel specifiers

Example:

raid_mbox: mbox@67400000 {

	...

	#mbox-cells = <3>;

	...

};

raid0 {

	compatible = "brcm,iproc-sba-v2";

	mboxes = <&raid_mbox 0 0x1 0xffff>,

		 <&raid_mbox 1 0x1 0xffff>,

		 <&raid_mbox 2 0x1 0xffff>,

		 <&raid_mbox 3 0x1 0xffff>,

		 <&raid_mbox 4 0x1 0xffff>,

		 <&raid_mbox 5 0x1 0xffff>,

		 <&raid_mbox 6 0x1 0xffff>,

		 <&raid_mbox 7 0x1 0xffff>;

};