bindings: iio: adc: Add bindings for TI ADS131M0x ADCs

bindings: iio: adc: Add bindings for TI ADS131M0x ADCs

Add device tree bindings documentation for the Texas Instruments
ADS131M0x analog-to-digital converters. This family includes the ADS131M02,
ADS131M03, ADS131M04, ADS131M06, and ADS131M08 variants.

These variants differ primarily in the number of supported channels
(2, 3, 4, 6, and 8, respectively), which requires separate compatible
strings to validate the channel nodes.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>