dt-bindings: iio: adc: ti-ads1298: Add bindings

Bindings for the TI ADS1298 medical ADC. This device is
typically used for ECG and similar measurements. Supports data
acquisition at configurable scale and sampling frequency.

The device has so many options for connecting stuff, at this
point the bindings aren't nearly complete but partial bindings
are better than no bindings at all.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240216153020.485201-1-mike.looijmans@topic.nl
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>