dt-bindings: iio: io-channel-mux: add optional #io-channel-cells

Needed for in-kernel use of the child channels of the mux.

Fixes problems like this, reported by dtbs_check:
adc0mux: '#io-channel-cells' does not match any of the regexes: 'pinctrl-[0-9]+'
	arch/arm/boot/dts/aspeed-bmc-ampere-mtjade.dt.yaml

Suggested-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>