iio: adc: ad7606: add 'bits' parameter to channels macros

iio: adc: ad7606: add 'bits' parameter to channels macros

There are some newer additions to the AD7606 family, which support 18 bit
precision. Up until now, all chips were 16 bit.

This change adds a 'bits' parameter to the AD760X_CHANNEL macro and renames
'ad7606_channels' -> 'ad7606_channels_16bit' for the current devices.

The AD7606_SW_CHANNEL() macro is also introduced, as a short-hand for IIO
channels in SW mode.

Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20240919130444.2100447-2-aardelean@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>