hwmon: (pmbus/core) Optimize debugfs block data attribute initialization

Define debugfs attributes which need block data access 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.

While this is currently only used for manufacturer specific attributes,
the access code is generic and also works for other block attributes,
so rename operation functions from _mfg to _block.

While at it, rename the "revison" file to "pmbus_revision" to make its
meaning more obvious and to create a clear distinction against the
"mfg_revision" file.

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