mei: export fw status registers through sysfs

The interface is for applications that monitor
the fw health.

We use device_create_with_groups interface
to register attribute with the mei class device

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>