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>