iio: light: rpr0521 proximity offset read/write

Add sysfs read/write proximity offset feature. Offset is read/write from
sensor registers. Values are proximity raw 10-bit values. After applying
offset value, output values will be (measured_raw - offset_value). Output
values are unsigned so offset value doesn't make output negative.

Signed-off-by: Mikko Koivunen <mikko.koivunen@fi.rohmeurope.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>