regulator: tps65910: Convert to use regulator_map_voltage_ascend

All regulators have ascendant voltage list in this driver.
Some regulators have more than 200 supported voltages.
e.g.
For TPS65910_REG_VDD1 and TPS65910_REG_VDD2:
n_voltages = VDD1_2_NUM_VOLT_FINE * VDD1_2_NUM_VOLT_COARSE
           = 73 * 3
           = 219

Thus it worth converting to regulator_map_voltage_ascend rather than use
default regulator_map_voltage_iterate.

For consistent, convert all regulators to regulator_map_voltage_ascend.

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