RDMA/efa: Add the efa.h header file

Add EFA driver generic header file defining driver's device independent
internal data structures and definitions.

Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>