The AMS driver DT binding was missing the clock entry, which is actually
mandatory according to the driver implementation. Add this in.

Fixes: 39dd2d1e ("dt-bindings: iio: adc: Add Xilinx AMS binding documentation")
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20220127173450.3684318-2-robert.hancock@calian.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>