dmaengine: fsl_raid: make of_device_ids const.

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   3981	    608	      0	   4589	   11ed	drivers/dma/fsl_raid.o

File size after constify:
   text	   data	    bss	    dec	    hex	filename
   4381	    192	      0	   4573	   11dd	drivers/dma/fsl_raid.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>