regulator: tps80031: Implement list_voltage and set n_voltages = 1 for fixed regulators

Implement list_voltage for fixed regulators, otherwise
regulator_is_supported_voltage() returns 0.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>