regulator: da9055: Properly handle voltage range that doesn't start with 0 offset

This patch implements map_voltage and list_voltage callbacks to properly handle
the case voltage range that doesn't start with 0 offset.

Now we adjust the selector in map_voltage() before calling set_voltage_sel().
And return 0 in list_voltage() for invalid selectors.

With above change, we can remove da9055_regulator_set_voltage_bits function.

One tricky part is that we need adding voffset to n_voltages.
Although for the cases "selector < voffset" are invalid, we need add voffset to
n_voltage so regulator_list_voltage() won't fail while checking the boundary for
selector before calling list_voltage callback.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Ashish Jangam <ashish.jangam@kpitcummins.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>