iwlwifi: add debugfs file to read fw debug data recording

FW debug data will oneshot read all data available in DRAM
and fill the supplied user buffer. In case the read request
is greater than the new data in DRAM, the driver will write
all data it has and return the buffer immediately.

Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Lior Cohen <lior2.cohen@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>