hwmon: (pmbus/core) Optimize debugfs status attribute initialization

Define debugfs attributes used to access status registers in a data
structure and loop through it instead of creating debugfs files
one by one. This reduces code size and simplifies adding additional
attributes if needed.

Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>