ARM i.MX6q: fix for ldb_di_sels

As pll5_video_div has been introduced to represent the clock
generated from post-divider for video.
Instead of pll5_video, pll5_video_div should be proper root clock
for ldb_di_sel.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>