hw: adjust priority to scheduling group map heuristic

[skalk]

Currently, the function to map Genode's internally used "virtual" priority values to a concrete scheduling group of the hw kernel behaves contradictory.
On the one hand, all priorities of the lower half, which is priority=0 in the top-level init (with two priority bands) of Sculpt, are getting mapped to the "driver" scheduling group. Whereby, all higher priorities have to exactly match the beginning of their corresponding priority bands to be mapped to the right scheduling group. Everything in between (imagine further sub-inits with priorities in Sculpt's runtime) are mapped to the background group.
Instead, it would be more intuitive to place all priorities matching one band into the same scheduling group.