iio: Introduce iio_device_{set,get}_drvdata()

Introduce two new helper functions to attach a arbitrary pointer to a IIO
device. This is useful to get access to external non-global data from within a
IIO device callbacks where only the IIO device is available.

Internally these functions use dev_{set,get}_drvdata() on the struct device
embedded in the IIO device.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>