IIO: Move the core files to drivers/iio

Take the core support + the kfifo buffer implentation out of
staging.  Whilst we are far from done in improving this subsystem
it is now at a stage where the userspae interfaces (provided by
the core) can be considered stable.

Drivers will follow over a longer time scale.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index d236aef..0265cb1 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -140,4 +140,6 @@
 
 source "drivers/devfreq/Kconfig"
 
+source "drivers/iio/Kconfig"
+
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