net/ethoc: support private memory configurations

The ethoc device can be configured to have a private memory region
instead of having access to the main memory. In that case egress packets
must be copied into that memory for transmission and pointers to that
memory need to be passed to net_process_received_packet or returned from
the recv callback.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>