iio: distance: srf08: add IIO driver for us ranger

This is the IIO driver for devantech srf08 ultrasonic ranger which can be
used to measure the distances to an object.

The sensor supports I2C with some registers.

Supported Features include:
 - read the distance in ranging mode in centimeters
 - output of the driver is directly the read value
 - together with the scale the driver delivers the distance in meters
 - only the first echo of the nearest object is delivered
 - set sensitivity as analog value in the range of 0-31  means setting
   gain register on device
 - set range registers; userspace enters max. range in millimeters in
   43 mm steps

Features not supported by this driver:
 - ranging mode in inches or in microseconds
 - ANN mode
 - change I2C address through this driver
 - light sensor

The driver was added in the directory "proximity" of the iio subsystem and
the menu in den config is now called "Proximity and distance sensors"

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>