Add missing SPDX License Identifier for driver-api documentation index
file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/driver-api/index.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index b208e0dac3a0..7a2584ab63c4 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -1,3 +1,5 @@
+.. SPDX-License-Identifier: GPL-2.0
+
 ==============================
 Driver implementer's API guide
 ==============================
-- 
2.25.1