crypto: sahara - 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
   9759	   2736	      8	  12503	   30d7	drivers/crypto/sahara.o

File size after constify:
   text	   data	    bss	    dec	    hex	filename
  10367	   2128	      8	  12503	   30d7	drivers/crypto/sahara.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>