dt-bindings: iio: adc: Add ast2600-adc bindings

Add device tree bindings document for the aspeed ast2600 adc device
driver.

Signed-off-by: Billy Tsai <billy_tsai@aspeedtech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210831071458.2334-3-billy_tsai@aspeedtech.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>