RDMA/core: Expose the ib port sysfs attribute machinery

RDMA/core: Expose the ib port sysfs attribute machinery

Other things outside the core code are creating attributes against the
port. This patch exposes the basic machinery to do this.

The ib_port_attribute type allows creating groups of attributes attatched
to the port and comes with the usual machinery to do this.

Link: https://lore.kernel.org/r/5c4aeae57f6fa7c59a1d6d1c5506069516ae9bbf.1623427137.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>