platform/x86: firmware_attributes_class: Create helper file for handling firmware-attributes class registration events

This offers shared code for registering the firmware_attributes_class,
which is used by the Dell and Lenovo WMI management drivers.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mark Pearson <markpearson@lenovo.com>
Link: https://lore.kernel.org/r/20210530223111.25929-1-markpearson@lenovo.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>