dt-bindings: iio: adc: Add AXP803 compatible

The AXP803 compatible was introduced recently with a fallback to the
AXP813, but it was never documented.

Cc: Jonathan Cameron <[e-mail]>
Cc: Lars-Peter Clausen <[e-mail]>
Cc: Peter Meerwald-Stadler <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Acked-by: Chen-Yu Tsai <[e-mail]>
Acked-by: Jernej Skrabec <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/iio/adc/x-powers,axp209-adc.yaml

···
 
 properties:
   compatible:
-    enum:
-      - x-powers,axp209-adc
-      - x-powers,axp221-adc
-      - x-powers,axp813-adc
+    oneOf:
+      - const: x-powers,axp209-adc
+      - const: x-powers,axp221-adc
+      - const: x-powers,axp813-adc
+
+      - items:
+          - const: x-powers,axp803-adc
+          - const: x-powers,axp813-adc
 
   "#io-channel-cells":
     const: 1