caif_shm: Add CAIF driver for Shared memory for M7400

caif_shm: Add CAIF driver for Shared memory for M7400

Add a caif shared memory link layer driver for ST-Ericsson's Thor M7400
LTE modem.

M7400 uses a ring-buffer in shared memory for transporting data from the modem.
Each ring-buffer element contains an array of caif frames. caif_shm calls
napi_schedule() when receiving notification about incoming data.
The napi-poll function copies data from the ring-buffer to SKBs until
ring-buffer is empty, or quota is exceeded.

If transmit ring-buffer is full, it also uses napi for scheduling transmission
of queued transmit buffer.

cc: David S. Miller <davem@davemloft.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>