Add support of xo-correction value

[naj1024]

Support for the xo_correction value for pluto devices would be useful.
Although I understand that it could lead to requests for support of all the other device end points as well.
On ad936x.py:

@property
def xo_correction(self):
    return self._get_iio_dev_attr("xo_correction")

@xo_correction.setter
def xo_correction(self, value):
    self._set_iio_dev_attr_str("xo_correction", value)