clk: meson8b: export the SAR ADC clocks

Export the clocks for the SAR ADC so they 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>