sandbox: ignore sandbox.dtb

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/.gitignore b/arch/sandbox/dts/.gitignore
new file mode 100644
index 0000000..b60ed20
--- /dev/null
+++ b/arch/sandbox/dts/.gitignore
@@ -0,0 +1 @@
+*.dtb