drm: sti: add VTG driver

Video Time Generator drivers are used to synchronize the compositor
and tvout hardware IPs by providing line count, sample count,
synchronization signals (HSYNC, VSYNC) and top and bottom fields
indication.
VTG are used by pair for each data path (main or auxiliary)
one for master and one for slave.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Rob Clark <robdclark@gmail.com>