[FeatureRequest] Add M code to configure CAN_0 and CAN_1 on 6HC/6XD

[droftarts]

Currently, only CAN_1 is supported, configured as a CAN_FD bus. Add options to:

1. Enable CAN_1 as CAN-FD bus, enable CAN_0 as CAN bus - This is the current default for 6HC/6XD, used by Hangprinter
2. Enable CAN_1 as CAN-FD bus, disable CAN_0 - possible in firmware
3. Enable CAN_0 as CAN-FD bus, disable CAN_1 - (dc42 said) There is a 6HC build configuration to use CAN0 instead of CAN1. It should still build, but hasn't been tested in a long time.
4. Enable CAN_0 as CAN-FD bus, enable CAN_1 as CAN bus - (dc42 said) Don't know whether plain CAN is supported on CAN1, it depends on how support was added for normal CAN on the second channel. So may be possible.

(dc42 said) CAN_0 and CAN_1 cannot be enabled as CAN-FD ports at the same time, because the firmware assumes all expansion boards are on the same bus.