iwlwifi: mvm: Add debugfs entry to generate NMI to NIC

Writting to fw_nmi entry will set DEVICE_SET_NMI_REG (0x00a01c30)
generating NMI that halts NIC CPU.

Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>