MIPS: dts: ar9331: fix interrupt-controller size

It is two registers each of 4 byte.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org