iio: ltr501: Export near level property for proximity sensor

Userspace tools like iio-sensor-proxy need to know the proximity level
that should be considered "near". This value is hardware-specific and
can be defined via the devicetree. Allow the driver to export the near
level.

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Link: https://lore.kernel.org/r/20211125125646.54831-2-nikita@trvn.ru


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