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/Kconfig b/drivers/iio/Kconfig
index b2f963be..daa3ddd 100644
--- a/drivers/iio/Kconfig
+++ b/drivers/iio/Kconfig
@@ -70,5 +70,8 @@
 source "drivers/iio/imu/Kconfig"
 source "drivers/iio/light/Kconfig"
 source "drivers/iio/magnetometer/Kconfig"
+if IIO_TRIGGER
+   source "drivers/iio/trigger/Kconfig"
+endif #IIO_TRIGGER
 
 endif # IIO