ixgbe: add initial devlink support

Add an initial support for devlink interface to ixgbe driver.

Similarly to i40e driver the implementation doesn't enable
devlink to manage device-wide configuration. Devlink instance
is created for each physical function of PCIe device.

Create separate directory for devlink related ixgbe files
and use naming scheme similar to the one used in the ice driver.

Add a stub for Documentation, to be extended by further patches.

Change struct ixgbe_adapter allocation to be done by devlink (Przemek),
as suggested by Jiri.

Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Co-developed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Bharath R <bharath.r@intel.com>
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>