pico-imx6ul: convert ethernet function to DM_ETH

pico-imx6ul: convert ethernet function to DM_ETH

- Remove pinmux definition from pico-imx6ul.c
- Enable NET_RANDOM_ETHADDR for temporary solution, because micrel_ksz8xxx
driver does not support DM_ETH yet, so cannot read MAC address directly.

Before enable DM_ETH:
  Net:   FEC [PRIME]

After enable DM_ETH:
  Net:
  Warning: using random MAC address - ca:3f:43:8f:67:d4
  eth1: ethernet@20b4000

Here is the test commands:
  => dhcp
  BOOTP broadcast 1
  DHCP client bound to address 10.88.88.94 (139 ms)
  *** ERROR: `serverip' not set
  Cannot autoload with TFTPGET
  => ping 8.8.8.8
  Using ethernet@20b4000 device
  host 8.8.8.8 is alive

Signed-off-by: Wig Cheng <wig.cheng@technexion.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>