dt-bindings: clk: meson-gxbb: Add Video clock bindings

Add the video clock bindings covering all the video graphics pipeline
and the HDMI controller.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: http://lkml.kernel.org/r/1541516257-16157-4-git-send-email-narmstrong@baylibre.com