configs: pico-imx6: convert ethernet function to DM_ETH

Before enable _DM_ETH:
    Net:   FEC [PRIME]

After enable DM_ETH:
    Net:   eth0: ethernet@2188000

Here is the test commands:
    => dhcp
    BOOTP broadcast 1
    DHCP client bound to address 10.88.88.152 (146 ms)
    *** ERROR: `serverip' not set
    Cannot autoload with TFTPGET
    => ping 8.8.8.8
    Using ethernet@2188000 device
    host 8.8.8.8 is alive

Signed-off-by: Wig Cheng <wig.cheng@technexion.com>
diff --git a/configs/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index 04c132b..5fe2850 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -64,6 +64,9 @@
 CONFIG_FSL_USDHC=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
+CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
+CONFIG_RGMII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
 CONFIG_MXC_UART=y