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: NZeng Zhaoming <zengzm.kernel@gmail.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>