iio: light: DT binding docs for APDS9960 driver

Document compatible string, and required DT properties for APDS9960
chipset driver.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>