iio: adc: aspeed: Reserve battery sensing channel for on-demand use

iio: adc: aspeed: Reserve battery sensing channel for on-demand use

For controllers with battery sensing capability (AST2600/AST2700), the
last channel uses a different circuit design optimized for battery
voltage measurement. This channel should not be enabled by default
along with other channels to avoid potential interference and power
efficiency issues.
This ensures optimal power efficiency for normal ADC operations while
maintaining full functionality when battery sensing is needed.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>