ata: sata_rcar: 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
   3946	   2296	      0	   6242	   1862	drivers/ata/sata_rcar.o

File size after constify sata_rcar_match.
   text	   data	    bss	    dec	    hex	filename
   5554	    696	      0	   6250	   186a	drivers/ata/sata_rcar.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>