ixgbe: Add support for entering low power link up state

When the device is closing or suspending, call ixgbe_enter_lplu to
enter low power link up state on devices that support it. When this
is done, prevent the phy from being reset in the ixgbe_down path
so that link is present when calling ixgbe_enter_lplu.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>