ARM: dts: aspeed: Add ADC for AST2600 and enable for Rainier and Everest

Add the ADC nodes to the AST2600 devicetree. Enable ADC1 for Rainier and
Everest systems and add an iio-hwmon node for the 7th channel to report
the battery voltage.

Tested on Rainier:
~# cat /sys/class/hwmon/hwmon11/in1_input
1347

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20210916210045.31769-1-eajames@linux.ibm.com


Signed-off-by: Joel Stanley <joel@jms.id.au>