configs: stih410-b2260: Enable DM_ETH flag

This patch allows to fix the following compilation 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: NPatrice Chotard <patrice.chotard@st.com>
Reviewed-by: NPatrick Delaunay <patrick.delaunay@st.com>

configs/stih410-b2260_defconfig

@@ -33,6 +33,7 @@ CONFIG_FASTBOOT_CMD_OEM_FORMAT=y
 CONFIG_MISC=y
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_STI=y
+CONFIG_DM_ETH=y
 CONFIG_PHY=y
 CONFIG_STI_USB_PHY=y
 CONFIG_PINCTRL=y