media: drivers/index.rst: add missing rkisp1 entry

The Documentation/driver-api/media/drivers/rkisp1.rst file
wasn't referenced in the index.rst file, so it was never included.

Add it.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>