dt-bindings: display: vc4: hdmi: 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. The documented order
was wrong too, so make sure clock-names is there and in the proper order.

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/10ef2821e10886b66af5f8ba3e212aa87e9fd360.1590594512.git-series.maxime@cerno.tech