dm: arm: rpi: Use driver model for Ethernet

Enable CONFIG_DM_ETH so that driver model is used for the USB Ethernet
device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_defconfig b/configs/rpi_defconfig
index 7ee2dee..d650e2e 100644
--- a/configs/rpi_defconfig
+++ b/configs/rpi_defconfig
@@ -16,6 +16,7 @@
 CONFIG_OF_EMBED=y
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_BCM2835=y
+CONFIG_DM_ETH=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_STORAGE=y