mlxsw: minimal: Add ethtool support

The minimal driver is chip independent and uses I2C bus for chip access.
Its purpose is to support chassis management on systems equipped with
Mellanox switch ASICs. For example from BMC (Board Management
Controller) device.

Expose a dummy netdev for each front panel port and implement basic
ethtool operations to obtain QSFP/SFP module info through ethtool.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>