epicardium: Query BLE hardware for idle periods

Query the BLE base band for idle periods and put the CPU to sleep during
this time. This allows us to reduce clock speed again and wake up in
time to quickly serve BLE interrupts.

In the future we can now even go lower than DIV4, as BLE continues to
work at least until DIV16.
2 jobs for !500 with schneider/ble-sleep in 1 minute and 57 seconds (queued for 9 seconds)
latest merge request