iio: proximity: stk3310: Export near level property for proximity sensor

This makes the value from which an object should be considered "near"
available to userspace. This hardware-dependent value should be set
in the device-tree.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>
Link: https://lore.kernel.org/r/20220420112540.91907-3-arnaud.ferraris@collabora.com


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