iio: adc: ti-ads1298: Add driver

Skeleton driver 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.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Link: https://lore.kernel.org/r/20240216153020.485201-2-mike.looijmans@topic.nl
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>