ASoC: wm8994: Support external capacitors on MICBIAS2 with jack detection

When an external capacitor is connected to MICBIAS2 on devices with
jack detection (which is not required but may be done in some systems)
then the loading may mean that better performance is obtained when
the microphone bias is enabled normally rather than using the low power
mode. Provide platform data allowing systems to indicate if they require
this.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>