regulator: Use tps65912_get_voltage for both DCDCs and LDOs

Now tps65912_get_voltage_dcdc and tps65912_get_voltage_ldo has exactly the same
implementation. We can merge them to tps65912_get_voltage function.

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