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>
diff --git a/configs/pico-imx6ul_defconfig b/configs/pico-imx6ul_defconfig
index 79c8d44..8ce0fe6 100644
--- a/configs/pico-imx6ul_defconfig
+++ b/configs/pico-imx6ul_defconfig
@@ -44,6 +44,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DFU_MMC=y
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_SIZE=0x10000000
@@ -57,6 +58,7 @@
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ8XXX=y
+CONFIG_DM_ETH=y
 CONFIG_MII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y