clk: meson: meson8b: mark clk81 as critical

Disabling clk81 results in an immediate freeze of the whole system. This
can happen "accidentally" when the last child-clock of clk81 is disabled
(in this case the common clock framework also disables clk81, even if it
was only enabled indirectly before).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>