ASoC: dt-bindings: wlf,wm8960: Describe the power supplies

WM8960 has the following power supplies:

- AVDD
- DBVDD
- DCVDD
- SPKVDD1
- SPKVDD1

Add them to bindings to improve the hardware description.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230803215506.142922-1-festevam@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>