dt-bindings: serial: 8250: Add Exar compatibles

The Intel IXP425 Vulcan board has an Exar XR16L2551 8250-compatible.
It works like an 8250 but it is always good to specify exactly which
component we are using. This allows us to specify:

compatible = "exar,xr16l2551", "ns8250";

Put in some sibling Exar serial compatibles while we're at it.

Cc: Marc Zyngier <maz@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210716225319.1282704-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/8250.yaml

@@ -64,6 +64,12 @@ properties:
       - const: nuvoton,npcm750-uart
       - const: nvidia,tegra20-uart
       - const: nxp,lpc3220-uart
+      - items:
+          - enum:
+              - exar,xr16l2552
+              - exar,xr16l2551
+              - exar,xr16l2550
+          - const: ns8250
       - items:
           - enum:
               - altr,16550-FIFO32