dt-bindings:iio:potentiometer:maxim,ds1803 move to trivial devices.

Simple binding where there is no obvious benefit in maintaining a
separate file.  Hence document in trivial-devices.yaml and drop
the txt file.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20201031182922.743153-4-jic23@kernel.org

Documentation/devicetree/bindings/iio/potentiometer/ds1803.txt

-* Maxim Integrated DS1803 digital potentiometer driver
-
-The node for this driver must be a child node of a I2C controller, hence
-all mandatory properties for your controller must be specified. See directory:
-
-        Documentation/devicetree/bindings/i2c
-
-for more details.
-
-Required properties:
-	- compatible:  	Must be one of the following, depending on the
-			model:
-			"maxim,ds1803-010",
-			"maxim,ds1803-050",
-			"maxim,ds1803-100"
-
-Example:
-ds1803: ds1803@1 {
-	reg = <0x28>;
-	compatible = "maxim,ds1803-010";
-};

Documentation/devicetree/bindings/trivial-devices.yaml

@@ -106,6 +106,12 @@ properties:
           - isil,isl68137
             # 5 Bit Programmable, Pulse-Width Modulator
           - maxim,ds1050
+            # 10 kOhm digital potentiometer with I2C interface
+          - maxim,ds1803-010
+            # 50 kOhm digital potentiometer with I2C interface
+          - maxim,ds1803-050
+            # 100 kOhm digital potentiometer with I2C interface
+          - maxim,ds1803-100
             # 10-bit 8 channels 300ks/s SPI ADC with temperature sensor
           - maxim,max1027
             # 10-bit 12 channels 300ks/s SPI ADC with temperature sensor