iwlwifi: mvm: add dbgfs entry for fw info

Add a dbgfs entry for an easy way during runtime to
check what FW file was loaded, and get some general
FW-related data.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>