Power-down support

Currently, disabling CTU CAN FD can be done by writing SETTINGS[ENA] = 0. If node is receiving, it can happen it will be cut-off in the middle of CAN frame, or or maybe even in the middle of ACK bit.

It would be convenient to add some sort of power-down/disabling support. The behavior shall be following:

1. User writes a bit to request power-down. CTU CAN FD captures this request.
2. CTU CAN FD waits for bus-idle and no transmissions ready. When it happens, it indicates that it can be disabled (in status register). Also, it automatically, pushes Protocol control logic to reset (as if SETTINGS[ENA] = 0).
3. User writes SETTINGS[ENA] = 0.