iio:adc:ad7280a: Document ABI for cell balance switches

Very minimal ABI docs. This is unusual enough that I'd expect anyone
who actually wanted to touch them to go look at the datasheet.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-18-jic23@kernel.org