dt-bindings: clock: meson8b: export the video decoder clocks

Export the four video decoder clocks so they can be used by the video
decoder driver:
- VDEC_1
- VDEC_HCODEC
- VDEC_2
- VDEC_HEVC

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20190324151423.19063-2-martin.blumenstingl@googlemail.com