net/uip: Add exit function

When resetting the virtio-net queues, the UIP state needs to be reset as
well. Stop all threads (one per TCP stream and UDP connection) and free
memory.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>