iio: Add iio.git tree to MAINTAINERS

The tree has been in the same location for a long time.  Putting it in
MAINTAINERS makes it easy for those new to, or less familiar with IIO
to find the correct tree to base patches on.

Mostly basing on staging-next is fine as well unless working on a very
active driver.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>