NFC: nfcsim: Make use of the Digital layer

With this complete rewrite, the loopback nfcsim driver now relies on the
Digital layer of the nfc stack. As with the previous version, 2 nfc
devices are declared when the driver is initialized. The driver supports
the NFC_DEP protocol in NFC-A and NFC-F technologies.

The 2 devices are using a pair of virtual links for sk_buff exchange.
The out-link of one device is the in-link of the other and conversely.

To receive data, a device calls nfcsim_link_recv_skb() on its in-link
and waits for incoming data on a wait queue. To send data, a device
calls nfcsim_link_send_skb() on its out-link which stores the passed skb
and signals its wait queue. If the peer device was in the
nfcsim_link_recv_skb() call, it will be signaled and will be able to
pass the received sk_buff up to the Digital layer.

Signed-off-by: Thierry Escande <[email protected]>
Signed-off-by: Samuel Ortiz <[email protected]>