samples: boards: nrf: battery: fix usage without voltage divider

The nRF battery sample claims that it works without the need of a vbatt
node (voltage divider). However, this was currently not possible because
the sample code did have no means to extract the necessary ADC details
(instance and channel) from Devicetree unless vbatt was present. Similar
to other ADC samples, the zephyr,user node is used to define io-channels
property in this case. The sample README has been updated with an
example of how measurement without a voltage divider can be achieved.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>