net: socketcan: decouple SocketCAN and CAN controller headers

Decouple the zephyr/net/socketcan.h and zephyr/drivers/can.h header files
by moving the SocketCAN utilities to their own header.

This is preparation for including the SocketCAN types defined in
socketcan.h in a native posix (Linux) SocketCAN driver context without name
clashes.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>