net: fec: add eee mode tx lpi support

The i.MX8MQ ENET version support IEEE802.3az eee mode, add
eee mode tx lpi enable to support ethtool interface.

usage:
1. set sleep and wake timer to 5ms:
ethtool --set-eee eth0 eee on tx-lpi on tx-timer 5000
2. check the eee mode:
~# ethtool --show-eee eth0
EEE Settings for eth0:
        EEE status: enabled - active
        Tx LPI: 5000 (us)
        Supported EEE link modes:  100baseT/Full
                                   1000baseT/Full
        Advertised EEE link modes:  100baseT/Full
                                    1000baseT/Full
        Link partner advertised EEE link modes:  100baseT/Full

Note: For realtime case and IEEE1588 ptp case, it should disable
EEE mode.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>