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/arch/nios2/dts/.gitignore b/arch/nios2/dts/.gitignore
new file mode 100644
index 0000000..b60ed20
--- /dev/null
+++ b/arch/nios2/dts/.gitignore
@@ -0,0 +1 @@
+*.dtb