ARM: dts: dh-imx6: Refactor DT with som and board level defs for use with imx6 duallite

ARM: dts: dh-imx6: Refactor DT with som and board level defs for use with imx6 duallite

To use the device tree definitions imx6q-dhcom-som.dtsi for all imx6 derivatives rename
it to imx6qdl-dhcom.dtsi. We omit the '-som', because it simplifies further mainlinening
of already existing device trees.
To reuse board level common stuff imx6qdl-dhcom-pdk2.dtsi is created and included by
imx6q-dhcom-pdk2.dts.

Signed-off-by Ludwig Zenz <lzenz@dh-electronics.com>
Reviewed-by: Marek Vasut <marex@denx.de>