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

The BMC can read the RTC battery voltage via ADC
channel 12.

Signed-off-by: Matt Spinler <spinler@linux.vnet.ibm.com>
Reviewed-by: Lei YU <mine260309@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>