MAINTAINERS: Add entry for Analog Devices IIO drivers

Add Michael and myself as the maintainer for the Analog Devices IIO drivers.
The entry matches on all files in drivers/staging/iio and drivers/iio/ starting
with the 'ad' prefix, except for 'adjd' as that one is used by Avago
Technologies.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>