dt-bindings: clk: meson: add ao slow clock path ids

Add the CLKIDs for the slow clock generation path

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lkml.kernel.org/r/20181221160239.26265-2-jbrunet@baylibre.com