dt-bindings: display: vc4: dpi: Add missing clock-names property

While the device tree and the driver expected a clock-names property, 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/8c7a4a9d85f7d1b23346367acf8d321ddad7cffe.1590594512.git-series.maxime@cerno.tech