iio: mxs-lradc: Move binding document out of staging as well

commit f836c45922446df872250a12dd08e48978aceb2f moved mxs-lradc driver
out of staging. However the binding document was left in the old place.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>