serrano: add support for panel color shifting based on a sysfs interface

Adapted from work by morfic and ktoonsez

Creates a syfs interface here:
  /sys/devices/virtual/sec/tsp/panel_colors

Will accept 0-4 for parameters:
  0 - Cold (more blues)
  1 - Cool
  2 - Normal (stock colors)
  3 - Warm
  4 - Hot (more reds)

Change-Id: I238f4bb0572c677b9c51de486e4274b54662db06