clk: meson: axg-audio: provide clk top signal name

The peripheral clock on the sm1 goes through some muxes
and dividers before reaching the audio gates. To model that,
without repeating our self too much, the "top" clock signal
is introduced and will serve as a the parent of the gates.

On the axg and g12a, the top clock is just a pass-through to
the audio peripheral clock provided by the main controller.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>