[media] dt/bindings: device tree description for AD5820 camera auto-focus coil

Add documentation for ad5820 device tree binding.
Signed-off-by: NPavel Machek <pavel@denx.de>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/devicetree/bindings/media/i2c/ad5820.txt

+* Analog Devices AD5820 autofocus coil
+
+Required Properties:
+
+  - compatible: Must contain "adi,ad5820"
+
+  - reg: I2C slave address
+
+  - VANA-supply: supply of voltage for VANA pin
+
+Example:
+
+       ad5820: coil@c {
+               compatible = "adi,ad5820";
+               reg = <0x0c>;
+
+               VANA-supply = <&vaux4>;
+       };
+