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>