net/mlx5_core: Introduce access function to read internal timer

A preparation step which adds support for reading the hardware
internal timer and the hardware timestamping from the CQE.
In addition, advertize device_frequency_khz HCA capability.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>