net: dsa: sja1105: make future_base_time a common helper

Because the PTP_CLK pin starts toggling only at a time higher than the
current PTP clock, this helper from the time-aware shaper code comes in
handy here as well. We'll use it to transform generic user input for the
perout request into valid input for the sja1105 hardware.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>