hwmon: (max31827) handle vref regulator

Add missing implementation for the max31827 supply regulator.
This is a hardware required property that is not handled.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20230925122929.10610-1-antoniu.miclaus@analog.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>