iwlwifi: support loading NVM data from file

Some newer devices will be integrated into the platform more
deeply and will not have embedded NVM (EEPROM/OTP). To support
such devices the NVM data must be provided by the platform,
allow loading the data via request_firmware() and then send it
to the device as needed.

Signed-off-by: Eran Harary <eran.harary@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>