Kconfig: Move CONFIG_DESIGNWARE_ETH to Kconfig

Move this to Kconfig and clean up board config files that use it. Also
rename it to CONFIG_ETH_DESIGNWARE to fit with the naming that exists
in drivers/net/Kconfig.

Signed-off-by: Simon Glass <sjg@chromium.org>
Version 1:
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/mixtile_loftq_defconfig b/configs/mixtile_loftq_defconfig
index 47453cd..8275df8 100644
--- a/configs/mixtile_loftq_defconfig
+++ b/configs/mixtile_loftq_defconfig
@@ -19,3 +19,6 @@
 CONFIG_USB1_VBUS_PIN="PH24"
 # No Vbus gpio for usb2
 CONFIG_USB2_VBUS_PIN=""
+CONFIG_ETH_DESIGNWARE=y
+CONFIG_NETDEVICES=y
+CONFIG_NET=y