iio: Move IIO Dummy Driver out of staging

This patch moves the reference IIO dummy driver from drivers/staging/iio
into a separate folder, drivers/iio/dummy and adds the proper Kconfig
and Makefile for it.

A new config menu entry called IIO dummy driver has also been added
in the Industrial I/O support menu, corresponding to this driver.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
index aeca726..6769f2f 100644
--- a/drivers/iio/Makefile
+++ b/drivers/iio/Makefile
@@ -16,6 +16,7 @@
 obj-y += chemical/
 obj-y += common/
 obj-y += dac/
+obj-y += dummy/
 obj-y += gyro/
 obj-y += frequency/
 obj-y += humidity/