iio:trigger:sysfs Move out of staging.

This simple driver is rather useful. No issues about its interface
have been raised for some time hence the proposal to move it out
of staging.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
index a0e8cdd..a349a96 100644
--- a/drivers/iio/Makefile
+++ b/drivers/iio/Makefile
@@ -21,3 +21,4 @@
 obj-y += imu/
 obj-y += light/
 obj-y += magnetometer/
+obj-y += trigger/