mx6ul(l)evk: Enable DM_ETH_PHY

mx6ul(l)evk: Enable DM_ETH_PHY

This fixes eth0 not initializing properly. Both eth0 and eth1 use the
MDIO interface on eth1 to access their PHYs. However, without
DM_ETH_PHY, eth0 still tries to use its own MDIO interface to access its
phy. Before this patch, a boot log might contain lines like

Net:
Warning: ethernet@20b4000 using MAC address from ROM
eth1: ethernet@20b4000 [PRIME]Could not get PHY for FEC0: addr 2

After this patch they now look like

Net:
Warning: ethernet@20b4000 using MAC address from ROM
eth1: ethernet@20b4000 [PRIME]Get shared mii bus on ethernet@2188000

Warning: ethernet@2188000 using MAC address from ROM
, eth0: ethernet@2188000

As an alternative to DM_ETH_PHY, FEC_MXC_SHARE_MDIO could also have been
used to achieve the same effect.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>