regulator: add regulator_desc_list_voltage_linear_range

Add regulator_desc_list_voltage_linear_range which can be used
by drivers for getting the voltages before regulator is registered.
This may be useful for drivers which need to fetch the voltage
selectors at device-tree parsing callback.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Mark Brown <broonie@kernel.org>
Tested-by: Angus Ainslie <angus@akkea.ca>
Reviewed-by: Angus Ainslie <angus@akkea.ca>
Signed-off-by: Mark Brown <broonie@kernel.org>