ASoC: Intel: cht-bsw-rt5672: Add key-mappings for the headset buttons

Having the headset buttons send BTN_0, BTN_1 and BTN_2 events is not
really useful. Add mappings to PLAYPAUSE VOLUME_UP and VOLUME_DOWN like
we do in other Intel machine drivers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>