[2/2] proc fs implementation for modifying kernel space from user space
It create /proc/a2020_gain; usage: echo "0 255" > /proc/a2020_gain
where 0 is audio path(only 0 for now), 255 - gain, its only applicable for DAC1(digital-analog converter) now.

Change-Id: I83c2f52a0bcb0fb09a7db0ebbad3067eb15be694