ARM: dts: imx6q: add missing links between ipu2 and mipi dsi

The backlinks are already there since commit 4520e692 ("ARM: dts:
imx6qdl: Add IPU DI ports and endpoints, move imx-drm node to dtsi")
and were moved by commit 70c2652c

 ("ARM: dts: imx6qdl: Move existing
MIPI DSI ports into a new 'ports' node"), but the links from IPU2 DI0/1
to the MIPI DSI mux are missing. Fix this.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>