net: marvell: prestera: Add ethtool interface support

The ethtool API provides support for the configuration of the following
features: speed and duplex, auto-negotiation, MDI-x, forward error
correction, port media type. The API also provides information about the
port status, hardware and software statistic. The following limitation
exists:

    - port media type should be configured before speed setting
    - ethtool -m option is not supported
    - ethtool -p option is not supported
    - ethtool -r option is supported for RJ45 port only
    - the following combination of parameters is not supported:

          ethtool -s sw1pX port XX autoneg on

    - forward error correction feature is supported only on SFP ports, 10G
      speed

    - auto-negotiation and MDI-x features are not supported on
      Copper-to-Fiber SFP module

Co-developed-by: Andrii Savka <andrii.savka@plvision.eu>
Signed-off-by: Andrii Savka <andrii.savka@plvision.eu>
Co-developed-by: Serhiy Boiko <serhiy.boiko@plvision.eu>
Signed-off-by: Serhiy Boiko <serhiy.boiko@plvision.eu>
Signed-off-by: Vadym Kochan <vadym.kochan@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>