regulator: Implement tps65912_list_voltage to be shared by both DCDCs and LDOs

Merge tps65912_list_voltage_dcdc and tps65912_list_voltage_ldo to
tps65912_list_voltage.  This change does not add too much complexity in
tps65912_list_voltage function.

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