iio: ad_sigma_delta: Allow to provide custom data register address

Some newer devices from the Sigma-Delta ADC family do have their data
register at a different address than the current default address. Add a
parameter to the ad_sigma_delta_info struct which allows to override the
default address.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>