msm: vidc: Add description of LTR Extradata

The missing string descriptor caused a mismatch in array sizes
causing the v4l2 control framework make an out of bounds access.

Change-Id: I9a9f64e16ca95163d800702d94528833f5892537
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>