Documentation: devlink rate objects

Add devlink rate objects section at devlink port documentation.
Add devlink rate support info at netdevsim devlink documentation.

Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>