dt-bindings: iio: st,stm32-adc: add top-level constraints

Properties with variable number of items per each device are expected to
have widest constraints in top-level "properties:" block and further
customized (narrowed) in "if:then:".  Add missing top-level constraints
for clock-names.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240818172951.121983-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>