clk: meson8b: export the gate clock for the HW random number generator

This exports the clock so it can be used in the dt-bindings.

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