hwmon: (pmbus) Add list_voltage to pmbus ops

When checking if a regulator supports a voltage range, the regulator
needs to have a list_voltage callback set to the regulator_ops or else
-EINVAL will be returned. This support does not exist for the pmbus
regulators, so this patch adds pmbus_regulator_list_voltage to the
pmbus_regulator_ops.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20220614093856.3470672-3-marten.lindahl@axis.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>