net: tracing: Add RX time tracing

RX time tracing tells how long it took from network packet
creation to when the stack got rid of it.
So the network device driver allocates net packet, this is the
start time. The end time is when the packet is fully processed.
Currently the limitation is that the RX time duration is used
for network packets that are tied to an open socket.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>