dt-bindings: iio: afe: current-sense-shunt: add io-channel-cells

The current-sense-shunt is an IIO provider thus can be referenced by IIO
consumers (via "io-channels" property in consumer device node).
Such provider is required to describe number of cells used in phandle
lookup with "io-channel-cells" property.  This also fixes dtbs_check
warnings like:

  arch/arm/boot/dts/s5pv210-fascinate4g.dt.yaml: current-sense-shunt:
    '#io-channel-cells' does not match any of the regexes: 'pinctrl-[0-9]+'

Fixes: ce66e52b ("dt-bindings:iio:afe:current-sense-shunt: txt to yaml conversion.")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>