ARM: dts: imx6qdl-dhcom: Move IPU iomux node from PDK2 to SoM file

The SoM itself provides the display interface, see [1] page 20.
Those pins have to be used as the RGB/DPI interface or not used
at all. So rather than duplicate the pinmux settings in every
carrier board DT, better move them into the SoM DTSI.

[1] https://wiki.dh-electronics.com/images/2/2e/DOC_DHCOM-Standard-Specification_R01_2016-11-17.pdf

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>