ASoC: Support WM8958 direct microphone detection IRQ

Allow direct routing of the WM8958 microphone detection signal to a GPIO
to be used, saving the need to demux the interrupt.

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