dt-bindings:iio:light:avago,apds9960: txt to yaml conversion

Very simple binding that we could move into trivial-devices.yaml
with a small loss of documentation.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20201031184854.745828-15-jic23@kernel.org