iio: adc: ad799x: Implement selecting external reference voltage input on AD7991, AD7995 and AD7999.

Make use of the AD7991_REF_SEL bit and support using the external
reference voltage if 'vref-supply' is present. Use VCC voltage supply
as reference if no extra reference is supplied.
Signed-off-by: Florian Boor <florian.boor@kernelconcepts.de>
Link: https://lore.kernel.org/r/20210930104249.2924336-1-florian.boor@kernelconcepts.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>