ARM: dts: imx7-mba7: add iio-hwmon support

Enable IIO hwmon support for ADC1 and ADC2. All channels are
available on X23 (ADC2) and X24 (ADC1) of MBa7x.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>