MAINTAINERS: Add entry for BMIPS multiplatform kernel

Add myself as a maintainer for the new BMIPS target.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: jfraser@broadcom.com
Cc: dtor@chromium.org
Cc: tglx@linutronix.de
Cc: jason@lakedaemon.net
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8505/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>