net-caif: add CAIF protocol definitions

Add CAIF definitions to existing header files.
Files: if_arp.h, if_ether.h, socket.h.
Types: ARPHRD_CAIF, ETH_P_CAIF, AF_CAIF, PF_CAIF, SOL_CAIF, N_CAIF

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