nios2: enable device tree control of U-Boot

This patch adds device tree control of U-Boot to nios2 boards.
The example dts is taken from Linux kernel.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/configs/nios2-generic_defconfig b/configs/nios2-generic_defconfig
index f8d2821..09cc51e 100644
--- a/configs/nios2-generic_defconfig
+++ b/configs/nios2-generic_defconfig
@@ -1,5 +1,6 @@
 CONFIG_NIOS2=y
 CONFIG_TARGET_NIOS2_GENERIC=y
+CONFIG_DEFAULT_DEVICE_TREE="3c120_devboard"
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_BOOTD is not set
 # CONFIG_CMD_IMLS is not set
@@ -10,4 +11,5 @@
 CONFIG_CMD_DHCP=y
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_PING=y
+CONFIG_OF_CONTROL=y
 CONFIG_NET_RANDOM_ETHADDR=y