docs: ptp.rst: Add information about NVIDIA Mellanox devices

The mlx5_core driver has implemented ptp clock driver functionality but
lacked documentation about the PTP devices. This patch adds information
about the Mellanox device family.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>