tools: iio: Add iio targets in tools Makefile

This patch adds targets for building and cleaning iio tools to tools/Makefile.
To build iio tools from the toplevel kernel directory one should call:

$ make -C tools iio

and for cleaning it

$ make -C tools iio_clean

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>