clk: meson: meson8b: make the hdmi_sys clock tree mutable

The HDMI TX controller requires the hdmi_sys clock to be enabled. Allow
changing the whole clock tree now that we know that one of our drivers
requires this.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20200330234535.3327513-3-martin.blumenstingl@googlemail.com