ACPI: acpi_pad: Use dev groups for sysfs

Change the way sysfs files are created. Use dev_groups, as it's a
better approach - it allows to declare attributes, and the core code
would take care of the lifecycle of those objects.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>