ARM: dts: imx: Simplify fec node for iMX6QDL DHCOM boards

Firstly the FEC can now use the regulator reg_eth_vio from
imx6qdl-dhcom-som.dtsi instead of defining its own.

Secondly the &fec node is moved to the more generic SoM device tree
file, because it can be used by multiple boards.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Philip Oberfichtner <pro@denx.de>