dt-bindings: iio: adc: ti,adc081c: Document the binding

dt-bindings: iio: adc: ti,adc081c: Document the binding

Linux has a driver for these ADCs at drivers/iio/adc/ti-adc081c.c, but
the compatible strings were undocumented. Add a binding for them. The
hardware has an alert interrupt output, but existing ti,adc081c users
do not provide the 'interrupts' property, so leave it as optional.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221125220903.8632-1-samuel@sholland.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>