Add "out-of-band" signals to internal bus interface

[stnolting]

The signals of the request bus are split in to two categories: in-band signals and out-of-band signals. In-band signals always belong to a certain bus transaction and are only valid between stb being set and the according response (err or ack). being set. In contrast, the out-of-band signals are not associated with any bus transaction and are always valid when set.

bus_req_t out-of-band signals

• fence - Data/instruction fence request; single-shot
• sleep - Set if ALL upstream devices are in sleep-mode
• debug - Set if the upstream device is in debug-mode