.gitignore: add SPDX License Identifier

Add SPDX License Identifier to all .gitignore files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arc/boot/.gitignore

@@ -1 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
 uImage