intel_mid_dma: Allow DMAC2 to share interrupt

Allow DMAC2 to share interrupt since exclusive interrupt line
for mrst DMAC2 is not provided on other platforms.

Signed-off-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>