docs: add SPDX tags to new index files

All those new files I added are under GPL v2.0 license.

Add the corresponding SPDX headers to them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

docs: early-userspace: move to driver-api guide

Those documents describe a kAPI. So, add to the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>