of/documentation: move video device bindings to a common place

Binding Documents for drm-devices are placed in
Documentation/devicetree/bindings/drm/*. But these devices are common
for v4l framework, hence moved to a common place at
Documentation/devicetree/bindings/video/. 'exynos_' prefix is added to
associate them with exynos soc series.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>