rpi3: Enable lan78xx driver

The new Raspberry Pi B 3+ has a lan78xx device attached to it. Let's add
driver support in U-Boot for it.

Signed-off-by: Alexander Graf <agraf@suse.de>

configs/rpi_3_defconfig

@@ -36,3 +36,5 @@ CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
 CONFIG_OF_LIBFDT_OVERLAY=y
+CONFIG_PHYLIB=y
+CONFIG_USB_ETHER_LAN78XX=y