ARM i.MX6DL: dts: add clock and mux configuration for LDB

i.MX6DL does not have the second IPU, but the LVDS multiplexers can connect
either LVDS channel of the LDB to IPU1 DI0 or IPU1 DI1.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
[shawn.guo: remove "crtcs" property from imx6qdl.dtsi]
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>