Add support for CAN-FD bitrate switching

[T3P3]

Optional for the system (i.e. done in config.g).

Related to
#991

[dc42]

We will need a facility for the main board to notify expansion boards whether BRS is to be used, and if so what the fast/normal data rate ratio is. One option for this is to always send time sync messages without using BRS and to use some of the 15 spare bits in the time sync message to specify whether BRS is in use and what the multiplier is. e.g. 0 = no BRS, 1 = x2, 2 = x3, 3 = x4. Alternatively, if we use a fixed ratio e.g. x4 then we can enable BRS on time sync messages too.