loader: Add a list of firmware name mapping

Since we started to ship raw firmware for iwm(4), users who loads
the driver from loader are having problems as loader don't know that
the firmwares are now raw files and not kernel modules anymore.
Start a list of default entry for iwm(4) firmwares name mapping so it will
still works when loaded from loader.

Differential Revision: https://reviews.freebsd.org/D48211
Reviewed by: bz, imp, kevans
Sponsored by: Beckhoff Automation GmbH & Co. KG