dt-bindings: adc: ltc2497: add support for ltc2305

dt-bindings: adc: ltc2497: add support for ltc2305

Add documentation for the 2-channel LTC2305 ADC in the
existing ltc2497 binding.
This enables automatic device tree matching for LTC2305
while using the LTC2309 driver (drivers/iio/adc/ltc2309.c),
since both ADCs share the same I2C interface and 12-bit SAR architecture.
The main difference is the number of channels (LTC2305: 2, LTC2309: 8).

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Kyle Hsieh <kylehsieh1995@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>