can: Documentation for the CAN device driver interface

This patch documents the CAN netowrk device drivers interface, removes
obsolete documentation and adds some useful links to CAN resources.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Oliver Hartkopp <oliver.hartkopp@volkswagen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>