net: txgbe: Register I2C platform device

Register the platform device to use Designware I2C bus master driver.
Use regmap to read/write I2C device region from given base offset.

Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>