dt-bindings:iio:potentiometer:maxim,max5481 move to trivial devices

Simple SPI binding that doesn't need a separate file.
During conversion I looked up the individual part number descriptions
in the datasheet so that we could give slightly more detail in
trivial-device.yaml.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Slawomir Stepien <sst@poczta.fm>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maury Anderson <maury.anderson@rockwellcollins.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20201031182922.743153-5-jic23@kernel.org

Documentation/devicetree/bindings/iio/potentiometer/max5481.txt

-* Maxim Linear-Taper Digital Potentiometer MAX5481-MAX5484
-
-The node for this driver must be a child node of a SPI controller, hence
-all mandatory properties described in
-
-        Documentation/devicetree/bindings/spi/spi-bus.txt
-
-must be specified.
-
-Required properties:
-	- compatible:  	Must be one of the following, depending on the
-			model:
-			"maxim,max5481"
-			"maxim,max5482"
-			"maxim,max5483"
-			"maxim,max5484"
-
-Example:
-max548x: max548x@0 {
-	compatible = "maxim,max5482";
-	spi-max-frequency = <7000000>;
-	reg = <0>; /* chip-select */
-};

Documentation/devicetree/bindings/trivial-devices.yaml

@@ -126,6 +126,14 @@ properties:
           - maxim,max1231
             # Low-Power, 4-/12-Channel, 2-Wire Serial, 12-Bit ADCs
           - maxim,max1237
+            # 10-bit 10 kOhm linear programable voltage divider
+          - maxim,max5481
+            # 10-bit 50 kOhm linear programable voltage divider
+          - maxim,max5482
+            # 10-bit 10 kOhm linear programable variable resistor
+          - maxim,max5483
+            # 10-bit 50 kOhm linear programable variable resistor
+          - maxim,max5484
             # PECI-to-I2C translator for PECI-to-SMBus/I2C protocol conversion
           - maxim,max6621
             # 9-Bit/12-Bit Temperature Sensors with I²C-Compatible Serial Interface