ARM: dts: meson8: add and use the real clock controller

This removes the dummy clk81 gate and replaces it with the actual clock
controller's CLKID_CLK81. This will also allow us to pass the real clock
IDs to all devices where the clock is controlled by clkc in the future.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>