dt-bindings: iio: adc: ad4130: Document interrupts property

dt-bindings: iio: adc: ad4130: Document interrupts property

The Data Ready/FIFO interrupt has a special behavior that inverts the
IRQ polarity when devices with FIFO support enter FIFO mode, while using
normal polarity for data ready.

Document the interrupts property to clarify this special behavior for
users.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>