hwmon: (tmp421) add support for defining labels from DT

hwmon: (tmp421) add support for defining labels from DT

tmp42x is a multichannel temperature sensor with several external
channels. Since those channels can be used to connect diodes placed
anywhere in the system, their meaning will vary depending on the
project. For this case, the hwmon framework has an idea of labels which
allows us to assign the meaning to each channel.

The similar concept is already implemented in ina3221 - the label for
each channel can be defined via device tree. See commit a9e9dd9c6de5
("hwmon: (ina3221) Read channel input source info from DT")

This patch adds support for similar feature to tmp421.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/dab0fda6ac0e8d7f163c3762a7fb1e595a4d8159.1634206677.git.krzysztof.adamski@nokia.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>