ARM: dts: mxs: add #io-channel-cells to mx28 lradc

Adding #io-channel-cells property to lradc allows us to use the lradc as
an iio provider.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>