netdevsim: implement support for devlink region and snapshots

Implement dummy region of size 32K and allow user to create snapshots
or random data using debugfs file trigger.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>