arc: gitignore: ignore ARC DTBs

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arc/dts/.gitignore b/arch/arc/dts/.gitignore
new file mode 100644
index 0000000..b60ed20
--- /dev/null
+++ b/arch/arc/dts/.gitignore
@@ -0,0 +1 @@
+*.dtb