hwmon: (pmbus/core) Add helper macro to define single pmbus regulator

The bindings for single instance regulator should be named with no instance
(e.g., buck not buck0). Introduce a new helper macro to define the single pmbus
regulator.

Signed-off-by: Saravanan Sekar <saravanan@linumiz.com>
Link: https://lore.kernel.org/r/20231011164754.449399-4-saravanan@linumiz.com


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