iommu/io-pgtable-arm-v7s: constify dummy_tlb_ops.

File size before:
   text	   data	    bss	    dec	    hex	filename
   6146	     56	      9	   6211	   1843	drivers/iommu/io-pgtable-arm-v7s.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6170	     24	      9	   6203	   183b	drivers/iommu/io-pgtable-arm-v7s.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>