drivers: regulator: pca9420: refactor voltage range handling

Voltage ranges were hardcoded in Devicetree, however, things can be
significantly simplified by using the recently introduce linear ranges
API. All values are now computed using information stored in the driver,
so there is no need to store any lookup table in ROM. Code should now
both be faster in average and consume less ROM.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>