mx6sabresd: Convert ethernet to driver model

Convert imx6sabresd ethernet to driver model to fix the following warning:

===================== WARNING ======================
This board does not use CONFIG_DM_ETH (Driver Model
for Ethernet drivers). Please update the board to use
CONFIG_DM_ETH before the v2020.07 release. Failure to
update by the deadline may result in board removal.
See doc/driver-model/migration.rst for more info.
====================================================

Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
diff --git a/configs/mx6sabresd_defconfig b/configs/mx6sabresd_defconfig
index 8629554..f218408 100644
--- a/configs/mx6sabresd_defconfig
+++ b/configs/mx6sabresd_defconfig
@@ -81,6 +81,11 @@
 CONFIG_SF_DEFAULT_SPEED=20000000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y
+CONFIG_PHY_ATHEROS=y
+CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
+CONFIG_FEC_MXC=y
+CONFIG_RGMII=y
 CONFIG_MII=y
 CONFIG_PCI=y
 CONFIG_DM_PCI=y