nios2: add .gitignore entries for auto-generated files

Add .gitignore entries for nios2 specific files generated during the
build process.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>
diff --git a/arch/nios2/boot/.gitignore b/arch/nios2/boot/.gitignore
new file mode 100644
index 0000000..109279c
--- /dev/null
+++ b/arch/nios2/boot/.gitignore
@@ -0,0 +1,2 @@
+*.dtb
+vmImage