ASoC: Allow userspace control of Speyside headphone output

In order to facilitate the widest range of use cases (especially things
like speakerphone) allow the headphone output to be enabled and disabled
by the application layer.

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