iio:pressure:ms5611: oversampling rate support

Add support for setting and retrieving OverSampling Rate independently for
each of the temperature and pressure channels. This allows userspace to
fine tune hardware sampling process according to the following tradeoffs :
* the higher the OSR, the finer the resolution ;
* the higher the OSR, the lower the noise ;
BUT:
* the higher the OSR, the larger the drift ;
* the higher the OSR, the longer the response time, i.e. less samples per
  unit of time.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>