ASoC: Disable WM8996 CPVDD supply when not in use

The WM8996 only requires CPVDD when the charge pump is active so control
it separately to the other supplies, only enabling it when the charge pump
is active. This will result in a small power saving on systems which are
able to provide independent software control of the supply.

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