mako: Add overlay for gamma control semantics

This change will allow for gamma control heading strings to be
set on a per-device basis, mako in this instance.

Change-Id: I51507e104a9f32a2f18359fa8151e0185d1644bc
Merge "mako: Add overlay for gamma control semantics" into cm-10.2
Merge branch 'cm-10.2' into martincz