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

drivers/hwmon/pmbus/pmbus.h

@@ -464,6 +464,7 @@ extern const struct regulator_ops pmbus_regulator_ops;
 #define PMBUS_REGULATOR(_name, _id)				\
 	[_id] = {						\
 		.name = (_name # _id),				\
+		.supply_name = "vin",				\
 		.id = (_id),					\
 		.of_match = of_match_ptr(_name # _id),		\
 		.regulators_node = of_match_ptr("regulators"),	\