ASoC: Display the power register in DAPM widget debugfs

Make it a bit easier to tie DAPM widgets in with the register map
without referring to the source by including the register location
controlled by the widget.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>