board: lsxl: convert to DM_ETH

Just enabling the Kconfig option for DM_ETH and DM_MDIO is enough.
Additionally, we can remove the old hardcoded config.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/configs/lsxhl_defconfig b/configs/lsxhl_defconfig
index d895312..6de0322 100644
--- a/configs/lsxhl_defconfig
+++ b/configs/lsxhl_defconfig
@@ -62,6 +62,8 @@
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SF_DEFAULT_SPEED=25000000
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_DM_ETH=y
+CONFIG_DM_MDIO=y
 CONFIG_MVGBE=y
 CONFIG_MII=y
 CONFIG_DM_REGULATOR=y