dt-bindings: adc: axi-adc: add clocks property

Add a required clock property as we can't access the device registers if
the AXI bus clock is not properly enabled.

Note this clock is a very fundamental one that is typically enabled
pretty early during boot. Independently of that, we should really rely on
it to be enabled.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Fixes: 96553a44

 ("dt-bindings: iio: adc: add bindings doc for AXI ADC driver")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240426-ad9467-new-features-v2-3-6361fc3ba1cc@analog.com
Cc: <Stable@ver.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>