ARM: dts: aspeed: cmm: enable iio-hwmon-adc

Bind aspeed ADC channels 0-7 to "iio-hwmon" driver so the data of these
voltage sensing channels can be accessed by "lm_sensors". Channels 8-15
are not used on CMM BMC.

Signed-off-by: Tao Ren <taoren@fb.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>