bq25890: Don't trigger ADC conversions too often

bq25890_power_supply_get_property triggers an ADC conversion on each read of every single property that's marked by bq25890_is_adc_property. This is wasteful, as reading all properties will cause multiple ADC conversion one after another. The driver could become more robust and don't re-query conversions if the last one happened during the last second or so.