dt-bindings: iio: adc: max1238: Add #io-channel-cells property

dt-bindings: iio: adc: max1238: Add #io-channel-cells property

Add #io-channel-cells property with a constant value of 1 because it is
multi-channel ADC.

Fix below CHECK_DTBS warnings:
  arch/arm/boot/dts/nxp/ls/ls1021a-iot.dtb: adc@35 (maxim,max1239): '#io-channel-cells' does not match any of the regexes: '^pinctrl-[0-9]+$'
	from schema $id: http://devicetree.org/schemas/iio/adc/maxim,max1238.yaml#

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250818200014.3700738-1-Frank.Li@nxp.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>