MAINTAINERS: add Documentation/iio/ to IIO subsystem

Patches touching the IIO subsystem documentation should also be sent to
the IIO mailing list.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240322-mainline-ad7944-doc-v2-1-0923d35d5596@baylibre.com


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