net/mlx5: convert Rx timestamps in real-time format

The ConnectX-6DX supports the timestamps in various formats,
the new realtime format is introduced - the upper 32-bit word
of timestamp contains the UTC seconds and the lower 32-bit word
contains the nanoseconds. This patch detects what format is
configured in the NIC and performs the conversion accordingly.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>