RDMA/efa: Add the efa module

Add the main EFA module file which takes care of device
probe/initialization/registration/etc.
Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Shiraz Saleem <shiraz.saleem@intel.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>