Docs/driver-api/index: Add missing SPDX License Identifier

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

Signed-off-by: SeongJae Park <sj@kernel.org>
Link: https://lore.kernel.org/r/20230122213650.187710-3-sj@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>