dt-bindings: serial: correct ref to serial.yaml

The serial bindings should reference the local (in kernel) serial.yaml,
not the /schemas/serial.yaml.  The latter comes from dtschema package
and is a small subset of serial bindings.

Usage of the local serial.yaml allows typical properties and children:

  xilinx/avnet-ultra96-rev1.dtb: serial@ff000000: Unevaluated properties are not allowed ('bluetooth' were unexpected)
Signed-off-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: NMichal Simek <michal.simek@amd.com>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230124091916.45054-2-krzysztof.kozlowski@linaro.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/cdns,uart.yaml

@@ -10,7 +10,7 @@ maintainers:
   - Michal Simek <michal.simek@xilinx.com>
 
 allOf:
-  - $ref: /schemas/serial.yaml#
+  - $ref: serial.yaml#
 
 properties:
   compatible:

Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml

@@ -63,7 +63,7 @@ required:
   - xlnx,use-parity
 
 allOf:
-  - $ref: /schemas/serial.yaml#
+  - $ref: serial.yaml#
   - if:
       properties:
         xlnx,use-parity: