hwmon: (pmbus) Add regulator supply into macro

Add regulator supply into PWBUS_REGULATOR macro. This makes it optional
to define a vin-supply in DT. Not defining a supply will add a dummy
regulator supply instead and only cause the following debug output:

```
Looking up vin-supply property in node [...] failed
```

Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
Link: https://lore.kernel.org/r/58f2ff7b90233fad3d7ae2e9d66d5192e2c1ac01.1645437439.git.sylv@sylv.io


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