video: Introduce video_sync operation

Some drivers like LCD connected via SPI requires explicit sync function
which copy framebuffer content over SPI to controller to display.
This hook doesn't exist yet that's why introduce it via video operations.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
2 files changed