e1000e: Implement system clock

e1000e: Implement system clock

The system clock is necessary to implement PTP features. While we are
not implementing PTP features for e1000e yet, we do have a plan to
implement them for igb, a new network device derived from e1000e,
so add system clock to the common base first.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>