ARM: dts: imx6q-dhcom: Use 1G ethernet on the PDK2 board

The PDK2 board is capable of running both 100M and 1G ethernet. However,
the i.MX6 has only one ethernet MAC, so it is possible to configure
either 100M or 1G Ethernet. In case of 100M option, the PHY is on the
SoM and the signals are routed to a RJ45 port. For 1G the PHY is on
the PDK2 board with another RJ45 port. 100M and 1G ethernet use
different signal pins from the i.MX6, but share the MDIO bus.

This SoM board combination is used to demonstrate how to enable 1G
ethernet configuration.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: kernel@dh-electronics.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>