drm: zte: add function to configure vou_ctrl dividers

The clock control module (CRM) cannot always provide desired frequency
for all VOU output devices.  That's why VOU integrates a few dividers
to further divide the clocks from CRM.  Let's add an interface for
configuring these dividers.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>