iio: adc: ad7476: Add AD7091 support

AD7091R is already supported by this driver. While AD7091R allows the
choice of an internal or an external voltage reference, for AD7091 the
reference is only provided by VDD. Since this information is anyway
obtained through the "vcc" regulator, no other driver changes are
required for adding AD7091 support as well.

Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>