iio: adc: ad7606: add enabling of optional Vrefin voltage

iio: adc: ad7606: add enabling of optional Vrefin voltage

Add optional refin voltage enabling. The property "refin-supply" is
already available and optional in the current fdt dt_schema.

Note that the driver does not need to take any actions if the supply
is not present because a pin strap is used to change the behavior
of the device if an external reference is connected.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250530-wip-bl-ad7606-reference-voltages-v2-2-d5e1ad7e6f14@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>