net: phy: add genphy_c45_ethtool_get/set_eee() support

Add replacement for phy_ethtool_get/set_eee() functions.

Current phy_ethtool_get/set_eee() implementation is great and it is
possible to make it even better:
- this functionality is for devices implementing parts of IEEE 802.3
  specification beyond Clause 22. The better place for this code is
  phy-c45.c
- currently it is able to do read/write operations on PHYs with
  different abilities to not existing registers. It is better to
  use stored supported_eee abilities to avoid false read/write
  operations.
- the eee_active detection will provide wrong results on not supported
  link modes. It is better to validate speed/duplex properties against
  supported EEE link modes.
- it is able to support only limited amount of link modes. We have more
  EEE link modes...

By refactoring this code I address most of this point except of the last
one. Adding additional EEE link modes will need more work.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>