iio: adc: add ltc2309 support

The LTC2309 is an 8-Channel, 12-Bit SAR ADC with an I2C Interface.

This implements support for all single-ended and differential channels,
in unipolar mode only.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20230828-ltc2309-v3-2-338b3a8fab8b@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>