iio: proximity: Add driver support for vcnl3020 proximity sensor

Proximity sensor driver based on light/vcnl4000.c code.
For now supports only the single on-demand measurement.

The VCNL3020 is a fully integrated proximity sensor. Fully
integrated means that the infrared emitter is included in the
package. It has 16-bit resolution. It includes a signal
processing IC and features standard I2C communication
interface. It features an interrupt function.

Datasheet: http://www.vishay.com/docs/84150/vcnl3020.pdf


Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>