can: Add documentation for CAN FD driver configuration

With Linux 3.15 the infrastructure for CAN FD hardware drivers had been
introduced into the kernel. Now the M_CAN driver and the peak_usb driver
support CAN FD. Update the documentation to show the latest CAN related
configuration options of 'ip' from iproute2 and describe the CAN FD specific
options to set the data bitrate and protocol version (ISO/non-ISO).

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>