ARM: dts: imx6qdl.dtsi: add "arm,shared-override" for pl310

The imx6 SMP system has the same DMA memory coherency issue [1] with
pl310 L2 controller. With this shared override bit set, the customer
reports the DMA coherency issue is gone. Besides, I have tested
the performance using USB ethernet with/without this bit, it shows
no difference.

[1] http://patchwork.ozlabs.org/patch/469362/Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6qdl.dtsi

@@ -185,6 +185,7 @@ L2: l2-cache@00a02000 {
 			cache-level = <2>;
 			arm,tag-latency = <4 2 3>;
 			arm,data-latency = <4 2 3>;
+			arm,shared-override;
 		};
 
 		pcie: pcie@0x01000000 {