Documentation: DT: Add "mti" vendor prefix

We have a bunch of platforms using "mti,cpu-interrupt-controller" but
the "mti" prefix isn't documented.  Fix this.

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/8169/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>