staging:iio:light:ltr659: Add detect threshold

staging:iio:light:ltr659: Add detect threshold

Proximity value changes since the different material on cover that affects IR
emit/receive. To generate threshold value, we move hand to 1cm distance from
the sensor, then read proximity value many times on physical platform to get
average value. This value is stored in DT, and export in the driver. HAL can
refer this value to judge if object is near/far from the sensor by reading
proximity value.

bug 1496946

Change-Id: I2d48c372f28c771cd77c94eb6002edb35b734db5
Signed-off-by: wahsu <wahsu@nvidia.com>
Reviewed-on: http://git-master/r/395159
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Sachin Nikam <snikam@nvidia.com>