In the upcoming patches we'll need to access the rest of the fields in
the punit power gating register, so prepare for that.

v2:
- add doc reference for the power well subsystem IDs (Jesse)
- remove IDs for non-existant DPIO_RX[23] subsystems (Jesse)
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>