staging:iio:light: Move tsl2x7x ABI docs to correct location.

These seem to be up to date but never moved out of staging when the driver
did. Hence let us move them out now.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Brian Masney <masneyb@onstation.org>
Cc: Brian Masney <masneyb@onstation.org>
Link: https://lore.kernel.org/r/20200905174721.216452-3-jic23@kernel.org