ARM: dts: aspeed: romulus: Enable iio-hwmon-battery

Add iio-hwmon-battery using adc channel 12 and enable adc to make
adc running. This channel is used to read RTC battery voltage.

Note with Romulus hardware design, it requires GPIOR3 to be pulled
high to read the voltage, otherwise the reading is 0.
When GPIOR3 is high, it consumes battery and impacts the battery life.
So it is left for user space to toggle the GPIO when trying to read the
voltage.

Signed-off-by: Lei YU <mine260309@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>