dt-bindings: display: vc4: dsi: Add missing clock properties

While the device tree and the driver expected a clock-names and a
clock-cells properties, it wasn't explicitly documented in the previous
binding. Make sure it is now.

Cc: devicetree@vger.kernel.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/867813ae6b3e9cff0e9627e6ed09569dee5573bc.1590594512.git-series.maxime@cerno.tech