igb: implement high frequency periodic output signals

In addition to interrupt driven target time output events, the i210
also has two programmable clock outputs.  These clocks support periods
between 16 nanoseconds and 140 milliseconds.  This patch implements
the periodic output function using the clock outputs when possible,
falling back to the target time for longer periods.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>