Bluetooth: Add combined LED trigger for controller power

Instead of just having a LED trigger for power on a specific controller,
this adds the LED trigger "bluetooth-power" that combines the power
states of all controllers into a single trigger. This simplifies the
trigger selection and also supports multiple controllers per host
system via a single LED.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>