iio: adc: ad799x: add reference voltage to ad7994

AD7994 supports external reference voltage on REFIN
pin so if a vref-supply has been defined it shall be
used.

Signed-off-by: Stefano Manni <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad799x.c

···
 	},
 	[ad7994] = {
 		.num_channels = 5,
+		.has_vref = true,
 		.noirq_config = {
 			.channel = {
 				AD799X_CHANNEL(0, 12),