regulator: Convert pcap-regulator to set_voltage_sel

After converting to set_voltage_sel, we can remove the workaroud of getting
the best match voltage for V1. The core will iterate through the whole voltage
table and find the best match for us.

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