net-caif: add CAIF core protocol stack header files

Add include files for the CAIF Core protocol stack.

caif_layer.h - Defines the structure of the CAIF protocol layers
cfcnfg.h     - CAIF Configuration Module for services and link layers
cfctrl.h     - CAIF Control Protocol Layer
cffrml.h     - CAIF Framing Layer
cfmuxl.h     - CAIF Muxing Layer
cfpkt.h	     - CAIF Packet layer (skb helper functions)
cfserl.h     - CAIF Serial Layer
cfsrvl.h     - CAIF Service Layer

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