e1000_regs: Add definitions for Intel 82574-specific bits

Signed-off-by: NDmitry Fleytman <dmitry.fleytman@ravellosystems.com>
Signed-off-by: NLeonid Bloch <leonid.bloch@ravellosystems.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>