igc: Add support for RX timestamping

This adds support for timestamping received packets.

It is based on the i210, as many features of i225 work the same way.
The main difference from i210 is that i225 has support for choosing
the timer register to use when timestamping packets. Right now, we
only support using timer 0. The other difference is that i225 stores
two timestamps in the receive descriptor, right now, we only retrieve
one.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>