iio: Move iio userspace applications out of staging

This patch moves iio userspace applications out of staging, to tools/iio/
and adds a Makefile in order to compile them easily. It also adds tools/iio/
to MAINTAINERS file.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>