regulator: axp20x-regulator: constify regulator_ops structure

Declare regulator_ops structure as const as it is only stored in the ops
field of a regulator_desc structure. This field is of type const, so
regulator_ops structures having this property can be made const too.

File size before: drivers/regulator/axp20x-regulator.o
   text	   data	    bss	    dec	    hex	filename
  16848	   1232	      0	  18080	   46a0 regulator/axp20x-regulator.o

File size after: drivers/regulator/axp20x-regulator.o
   text	   data	    bss	    dec	    hex	filename
  17888	    192	      0	  18080	   46a0 regulator/axp20x-regulator.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>