net: microchip: sparx5: add function for calculating PTP basetime

Add a new function for calculating PTP basetime, required by the stream
gate scheduler to calculate gate state (open / close).

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>