ARM: dts: mx23: fix iio-hwmon support

In order to get iio-hwmon support, the lradc must be declared as an
iio provider. So fix this issue by adding the #io-channel-cells property.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Fixes: bd798f9c ("ARM: dts: mxs: Add iio-hwmon to mx23 soc")
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>