regulator: anatop: Use linear_min_sel with linear mapping

By setting linear_min_sel to anatop_reg->min_bit_val, we can avoid
adjust the anatop_reg->min_bit_val offset in [set|get]_voltage_sel.

With this chance we can refactor this driver to use
regulator_[get|set]_voltage_sel_regmap.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Ying-Chun Liu <paulliu@debian.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>