regulator: tps80031: Constify regulator_ops and tps80031_dcdc_voltages array

These regulator_ops variables and tps80031_dcdc_voltages array never need
to be modified, make them const so compiler can put them to .rodata.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>