e1000: add smart power down code

Smart Power Down is a power saving feature in newer e1000 hardware. We
disable it because it causes time to link to be long, but make it a
user choice.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>