defconfig: am43xx_evm: enable eth driver model

Enable eth driver model for am43xx_evm as cpsw supports
driver model.

This was already added with the commit bc705ea1cf12 but with
commit 4c4e3b37750f to add fit support CONFIG_DM_ETH was missed.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index b3fe269..56782f1 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -56,3 +56,4 @@
 CONFIG_G_DNL_PRODUCT_NUM=0xbd00
 CONFIG_SPL_OF_LIBFDT=y
 CONFIG_DM_I2C=y
+CONFIG_DM_ETH=y