net/mlx5: Support one PTP device per hardware clock

Currently, mlx5 driver exposes a PTP device for each network interface,
resulting in multiple device nodes representing the same underlying
PHC (PTP hardware clock). This causes problem if it is trying to
synchronize to itself. For instance, when ptp4l operates on multiple
interfaces following different masters, phc2sys attempts to
synchronize them in automatic mode.

PHC can be configured to work as free running mode or real time mode.
All functions can access it directly. In this patch, we create one PTP
device for each PHC when it's running in real time mode. All the
functions share the same PTP device if the clock identifies they query
are same, and they are already grouped by devcom in previous commit.
The first mdev in the peer list is chosen when sending
MTPPS/MTUTC/MTPPSE/MRTCQ to firmware. Since the function can be
unloaded at any time, we need to use a mutex lock to protect the mdev
pointer used in PTP and PPS callbacks. Besides, new one should be
picked from the peer list when the current is not available.

The clock info, which is used by IB, is shared by all the interfaces
using the same hardware clock.

Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>