dt-bindings: iio: adc: sd-modulator: fix io-channel-cells

io-channel-cells should be <0> since sigma delta modulator exports only
one channel, as described in ../iio/iio-bindings.txt "IIO providers"
section. Only the phandle is necessary for IIO consumers in this case.

Fixes: af111437 ("IIO: Add DT bindings for sigma delta adc modulator")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>