dt-bindings: iio: light: stk33xx: Add proximity-near-level

This allows exporting the value from which userspace should assert an
object is "near".

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220420112540.91907-2-arnaud.ferraris@collabora.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>