Now all the callback implementation are the same for both ldo and dvm
regulator_ops, merge them to tps6586x_regulator_ops.

Also rename tps6586x_ldo_list_voltage to tps6586x_list_voltage.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>