iio: srf08: add sensor type srf10

Ultrasonic sensor srf10 is quite similar to srf08 and now also supported by
the driver as device tree compatible string.

It was necessary to prepare the source for supplementary sensors. This is
done by enum srf08_sensor_type.

The most significiant difference between srf08 and srf10 is another range
and values of register gain (in the driver it's call sensitivity).
Therefore the array of it is extended and dependent of the sensor type.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>