radeon has different case but same name firmware files. the lower case
ones are already installed into /libdata/firmware, and to avoid having
to provide some special method to build on systems with case-preserving
file systems (such as macOS), we install the lower case ones in /usr.
this means that anyone that requires the lower case files must either
have /usr as part of the / file system, or manually keep them updated.
not yet hooked into build.
part of PR#54976, though i doubt we'll try to pull up these.
these share the same license as amdgpu-firmware, so keep them in
the same license subdir.
skip importing the upper case ones that are already available
in sys/dev/microcode.
mrg