Seaboard derivate Kaen has a GPIO to mute the headphone output. Add a field
to tegra_wm8903_platform_data so the board files can pass the GPIO number
for that to the ASoC machine driver.

Also, initialize this new field to a "not present" value for Harmony.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>