ARM: dts: imx31: fix AVIC base address

On i.MX31 AVIC interrupt controller base address is at 0x68000000.

The problem was shadowed by the AVIC driver, which takes the correct
base address from a SoC specific header file.

Fixes: d2a37b3d ("ARM i.MX31: Add devicetree support")
Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>