net: lan966x: Add is2 vcap model to vcap API.

This provides the lan966x is2 model and adds it to the vcap control
instance that will be provided to the vcap API.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>