Implement {get,set}_voltage_sel, list_voltage, map_voltage with
the useful regulator regmap helpers in order to be able to manage
the voltage of LAB (positive) and IBB (negative) regulators.

In particular, the supported ranges are the following:
- LAB (pos):  4600mV to  6100mV with 100mV stepping,
- IBB (neg): -7700mV to -1400mV with 100mV stepping.
Signed-off-by: NAngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: NBjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210113194214.522238-2-angelogioacchino.delregno@somainline.orgSigned-off-by: NMark Brown <broonie@kernel.org>