ASoC: rk3399_gru_sound: Map BTN_0 to KEY_PLAYPAUSE

ASoC: rk3399_gru_sound: Map BTN_0 to KEY_PLAYPAUSE

The Android 3.5mm Headset jack specification mentions that BTN_0 should
be mapped to KEY_MEDIA, but this is less logical than KEY_PLAYPAUSE,
which has much broader userspace support.

For example, the Chrome OS userspace now supports KEY_PLAYPAUSE to toggle
play/pause of videos and audio, but does not handle KEY_MEDIA.

Furthermore, Android itself now supports KEY_PLAYPAUSE equivalently, as the
new USB headset spec requires KEY_PLAYPAUSE for BTN_0.
https://source.android.com/devices/accessories/headset/usb-headset-spec

Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>