MAINTAINERS: Add entry for BCM33xx cable chips

Add myself as a maintainer for the new BCM3384 board support code.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: mbizon@freebox.fr
Cc: jogo@openwrt.org
Cc: jfraser@broadcom.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8171/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>