dt-bindings: arm: fsl: Document i.MX6DL DHCOM SoM on PDK2 carrier board

dt-bindings: arm: fsl: Document i.MX6DL DHCOM SoM on PDK2 carrier board

Document the DH electronics i.MX6DL DHCOM 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, analog audio, PCIe
and HDMI video output.

All of the aforementioned features except for mSATA are supported, mSATA
is not available on i.MX6DL and is only available on DHCOM populated with
i.MX6Q SoC which is already supported upstream.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>