In the absence of a full LED framework, allow the LEDs to be toggled
on/off using gpiobus.
We do not support any brightness control currently, blinking, group
settings, etc.
The feature we do support, given I missed to toggle it on first pass
and stared at the blank darkness, is low power mode, so that the
osciallator stays off if no LEDs are on.

This allows us to at least signal some status using the LEDs present
on SoCs even if we cannot easily morse or disco.