power: supply: core: tabularize HWMON temperature labels

Rework power_supply_hwmon_read_string() to check it's parameters.
This allows to extend it later with labels for other types of
measurements.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>