Power-down support

[Blebowski]

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.

Migrated from GitLab: https://gitlab.fel.cvut.cz/canbus/ctucanfd_ip_core/-/issues/421

[Blebowski]

Simpler approach would be manipulation directly via SETTINGS[ENA].

Enabling:

When SETTINGS[ENA]=0, and user writes SETTINGS[ENA]=1, then SETTINGS[ENA] becomes 1
immediately. User polls on FAULT_STATE to get the moment in time when the unit joined
the bus.

I would not change enabling.

Disabling:

When SETTINGS[ENA]=1 and user writes SETTINGS[ENA]=0, the SETINGS[ENA] would stay
1 until nearest bus idle/intermission, and it would turn 0 only then. This would mean,
that if user/driver writes SETTINGS[ENA] in the middle of frame transmission, no
error frame.

This would allow "turning off" the core also without clean-up of TXT Buffers first.