mx6slevk_spl: Convert to DM_ETH

Convert to DM_ETH to avoid board removal from the project.
Signed-off-by: NFabio Estevam <festevam@gmail.com>

configs/mx6slevk_spl_defconfig

@@ -52,6 +52,9 @@ CONFIG_SF_DEFAULT_MODE=0
 CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y
+CONFIG_PHY_SMSC=y
+CONFIG_DM_ETH=y
+CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y