ARM: dts: imx6q: Add support for the DHCOM iMX6 SoM and PDK2

Add support for the DH i.MX6 Quad based SoM and a PDK2 evaluation board.
The evaluation board features three serial ports, USB OTG, USB host with
an USB hub, Fast or Gigabit ethernet, eMMC, uSD, SD, mSATA, analog audio,
PCIe and HDMI video output.

All of the aforementioned features are supported by this patch.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>