clk: meson8b: export the HDMI system clock

Export the HDMI system clock (used by the HDMI transmitter) so it can be
used in the dt-bindings.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200330234535.3327513-2-martin.blumenstingl@googlemail.com