dt-bindings: clock: meson8b: export the VPU clock

The VPU clock is an input the the "VPU" (Video Processing Unit), which is
one of the components of the display controller.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20190324151104.18397-2-martin.blumenstingl@googlemail.com