ARC: Add some .gitignore entries

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
diff --git a/arch/arc/boot/.gitignore b/arch/arc/boot/.gitignore
new file mode 100644
index 0000000..5d65b54
--- /dev/null
+++ b/arch/arc/boot/.gitignore
@@ -0,0 +1 @@
+*.dtb*