dt-bindings: iio: light: ltr501: Add proximity-near-level

This value inidcates the proximity level that should be considered
"close".

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211125125646.54831-1-nikita@trvn.ru


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