netdevsim: add software driver for testing offloads

To be able to run selftests without any hardware required we
need a software model.  The model can also serve as an example
implementation for those implementing actual HW offloads.
The dummy driver have previously been extended to test SR-IOV,
but the general consensus seems to be against adding further
features to it.

Add a new driver for purposes of software modelling only.
eBPF and SR-IOV will be added here shortly, others are invited
to further extend the driver with their offload models.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>