The whole point of supply widgets is that they aren't inputs to their
sinks so a state change in a supply should never affect the state of the
widget being supplied and we don't need to mark them as dirty.

           Power    Path   Neighbour
Before:    69       727    905
After:     63       607    731

This is particularly useful where supplies affect large portions of the
chip (eg, a bandgap supplying the analogue sections).
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>