arm64: dts: imx8mp: Add FEC RMII pin mux on i.MX8MP DHCOM

The i.MX8MP DHCOM SoM may come with either external RGMII PHY or
LAN8740Ai RMII PHY on the SoM attached to FEC MAC. Add pin mux
settings for both options, so that DT overlay can override these
settings on SoM variant with the LAN8740Ai PHY.

Signed-off-by: Marek Vasut <marex@denx.de>
diff --git a/arch/arm/dts/imx8mp-dhcom-pdk2.dts b/arch/arm/dts/imx8mp-dhcom-pdk2.dts
index 382fbed..ac104cd 100644
--- a/arch/arm/dts/imx8mp-dhcom-pdk2.dts
+++ b/arch/arm/dts/imx8mp-dhcom-pdk2.dts
@@ -117,7 +117,9 @@
 /delete-node/ &ethphy1f;
 
 &fec {	/* Second ethernet */
+	pinctrl-0 = <&pinctrl_fec_rgmii>;
 	phy-handle = <&ethphypdk>;
+	phy-mode = "rgmii";
 
 	mdio {
 		ethphypdk: ethernet-phy@7 { /* KSZ 9021 */