hwmon: (tmp421) support specifying n-factor via DT

hwmon: (tmp421) support specifying n-factor via DT

Previous patches added a way to specify some channel specific parameters
in DT and n-factor is definitely one of them. This calibration mechanism
is board specific as its value depends on the diodes/transistors being
connected to the sensor and thus the DT seems like a right fit for that
information. It is very similar to the value of shunt resistor that some
drivers allows specifying in DT.

This patch adds a possibility to set n-factor for each channel via
"n-factor" DT property in each channel subnode.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/69d0bfcc5ba27c67f21d3eabfb100656a14c75b9.1634206677.git.krzysztof.adamski@nokia.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>