dt-bindings: iio: adc: ti,ads1015: fix datarate max value and meaning

dt-bindings: iio: adc: ti,ads1015: fix datarate max value and meaning

Datarate (dr) is a 3-bit wide register field. Values from 0 to 7 are
allowed for all devices but only for the ADS1115 devices a value of 7
does make a difference.

While on it fix the description of the datarate for ADS1115 devices as
well.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230621160857.3400747-1-m.felsch@pengutronix.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>