RDMA/efa: Add the efa_com.h file

A helper header file for EFA admin queue, admin queue completion,
asynchronous notification queue, and various hardware configuration data
structures and functions.

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