devlink: Move devlink port function hw_addr attr documentation

devlink port function hw_addr attr documentation is in mlx5 specific
file while there is nothing mlx5 specific about it.
Move it to devlink-port.rst.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>