staging: iio: tsl2x7x: add device tree documentation

Add device tree documentation for the tsl2x7x IIO driver.

Signed-off-by: Brian Masney <masneyb@onstation.org>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: devicetree@vger.kernel.org
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>