infiniband: add a Documentation driver-api chapter for Infiniband

Add a driver-api chapter for InfiniBand interfaces.

Link: https://lore.kernel.org/r/20191010035240.310347906@gmail.com
Signed-off-by: Randy Dunlap <rd.dunlab@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>