ASoC: sgtl5000: Enable VAG when DAC/ADC up

As manual described, VAG is an internal voltage reference of DAC/ADC,
So enabled it before DAC/ADC up.

One more thing should care about is VAG fully ramped down requires 400ms,
wait it to avoid pop.

Signed-off-by: Zeng Zhaoming <zengzm.kernel@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>