ioat2,3: convert to a true ring buffer

Replace the current linked list munged into a ring with a native ring
buffer implementation.  The benefit of this approach is reduced overhead
as many parameters can be derived from ring position with simple pointer
comparisons and descriptor allocation/freeing becomes just a
manipulation of head/tail pointers.

It requires a contiguous allocation for the software descriptor
information.

Since this arrangement is significantly different from the ioat1 chain,
move ioat2,3 support into its own file and header.  Common routines are
exported from driver/dma/ioat/dma.[ch].

Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/ioat/Makefile

@@ -1,2 +1,2 @@
 obj-$(CONFIG_INTEL_IOATDMA) += ioatdma.o
-ioatdma-objs := pci.o dma.o dca.o
+ioatdma-objs := pci.o dma.o dma_v2.o dca.o