asoc/ab8500: implement headset volume control

To enable:
echo 1 > /sys/module/snd_soc_ab8500_audio/parameters/hsx_boosted

To disable:
echo 0 > /sys/module/snd_soc_ab8500_audio/parameters/hsx_boosted

P.S  Need to stop music for seconds (let ab8500 asoc powers off),and turn on music to take effect 

Enjoy! :)

Thanks Aditya(Adi_Pat) for hints!