docs: infiniband: add it to the driver-api bookset

While this contains some uAPI stuff, it was intended to be read by a
kernel doc. So, let's not move it to a different dir, but, instead, just
add it to the driver-api bookset.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>