arm: dart6ul: enable DM_ETH for the dart6ul

This patch converts the dart6ul ethernet support to DM_ETH and cleans
up the legacy ethernet code. The clean up, more specifically:

* moves the fec2 node and pin definition to the carrier board DTS
  since the phy associated with it is on the carrier board and not on
  the SoM;
* add the reset pin associated to each phy;
* separate the ethernet, mdio and reset pins of each fec so that they
  are easier to reference;
* add clock properties to the phy nodes since they are connected to the
  50Mhz ENET[12]_TX_CLK clock of the SoC;
* remove CONFIG_BOARD_EARLY_INIT_F since the function is now empty.

Signed-off-by: Marc Ferland <ferlandm@amotus.ca>