dt-bindings: iio: adc: update the doc for SAR ADC

Update the doc as the SAR ADC modules doesn't require "sana" clock.

Singed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>