dts: bindings: test: Add pin inversion to vnd,serial binding

Add the `uart-controller-pin-inversion.yaml` include to the `vnd,serial`
test binding. This enables testing for other drivers that require pin
inversion to function correctly. For example, the
`worldsemi,ws2812-uart` LED strip driver relies on `tx-invert`.

Signed-off-by: Wai-Hong Tam <waihong@google.com>

Author: waihongtam

dts/bindings/test/vnd,serial.yaml

@@ -2,7 +2,7 @@ description: Test serial port
 
 compatible: "vnd,serial"
 
-include: uart-controller.yaml
+include: [uart-controller.yaml, uart-controller-pin-inversion.yaml]
 
 properties:
   baud-rate: