igb: add basic runtime PM support

Use the runtime power management framework to add basic runtime PM support
to the igb driver. Namely, make the driver suspend the device when the link
is off and set it up for generating a wakeup event after the link has been
detected again. This feature is disabled by default.

Based on e1000e's runtime PM code.

Changes since v1:
Don't suspend the device when shutting down the interface.
Avoid race between runtime suspending and ethtool operations.

Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>