iio: add ad5761 DAC driver

ad5761 is a 1-channel DAC with configurable output range.
The driver uses the regulator interface for its voltage ref.

It shares its register layout with ad5761r, ad5721 and ad5721r.

Differences:
ad5761* are 16 bit, ad5721* are 12 bits.
ad57*1r have an internal reference.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/CREDITS b/CREDITS
index 8207cc6..44ea433 100644
--- a/CREDITS
+++ b/CREDITS
@@ -3035,6 +3035,7 @@
 D: PCA9634 driver
 D: Option GTM671WFS
 D: Fintek F81216A
+D: AD5761 iio driver
 D: Various kernel hacks
 S: Qtechnology A/S
 S: Valby Langgade 142