regulator: max1586: Implement get_voltage_sel callback

This is required since commit f7df20ec
"regulator: core: Use list_voltage() to read single voltage regulators",
otherwise _regulator_get_voltage returns rdev->desc->ops->list_voltage(rdev, 0).

The Maxim 1586 controls V3 and V6 voltages, but offers no way of reading back
the set up value. Thus this patch caches the setting when setting new voltage.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>