regulator: pwm: No need to make a copy of regulator_ops per instance

Having instance specific copy of desc is enough to support multiple
instance of pwm regulator.
The regulator_ops is never changed so no need to copy it per instance, make
pwm_regulator_voltage_table_ops and pwm_regulator_voltage_continuous_ops
const to ensure they won't be changed.
The pwm_regulator_desc is a template to be copied so also make it const.

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