imx: imx8mp_evk: enable eth support

Add board code to configure the network interface
Add net defconfig

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/dts/imx8mp-evk-u-boot.dtsi b/arch/arm/dts/imx8mp-evk-u-boot.dtsi
index 4f00b5a..0bf6ef5 100644
--- a/arch/arm/dts/imx8mp-evk-u-boot.dtsi
+++ b/arch/arm/dts/imx8mp-evk-u-boot.dtsi
@@ -139,3 +139,7 @@
 &wdog1 {
 	u-boot,dm-spl;
 };
+
+&fec {
+	phy-reset-gpios = <&gpio4 2 GPIO_ACTIVE_LOW>;
+};