Registered the mpi3mr driver source files in sys/conf/files, enabling
it to be compiled into the base kernel image. This matches the approach
used for existing Broadcom drivers such as mrsas, mps, and mpr.
Also updated sys/conf/NOTES with an entry for the mpi3mr driver.
With this change, the mpi3mr driver will be built as part of the base
kernel, allowing automatic loading during boot when supported hardware
is detected.