regulator: lp3971: Convert to use regulator_map_voltage_ascend

All regulators have ascendant voltage list in this driver.
Thus use regulator_map_voltage_ascend is more efficient than the default
regulator_map_voltage_iterate.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>