mpi3mr: Allow driver to be in-kenrel and add to GENERIC

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.

Add mpi3mr to sys/conf/NOTES.

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.

Below changes are suggested by chs@:

• Corrected the file name typo mistake in /sys/conf/files from mpi3mr_app.c.c to mpi3mr_app.c
• Added mpi3mr driver entries in "sys/amd64/conf/GENERIC" and "sys/i386/conf/GENERIC"

Reviewed by: ssaxena, chs, imp
Differential Revision: https://reviews.freebsd.org/D49754

(cherry picked from commit 9cdd40759617b15fdd6939d33f67aa2c9d2a6b1e)