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: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
---
 Documentation/devicetree/bindings/serial/cdns,uart.yaml         | 2 +-
 Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/serial/cdns,uart.yaml b/Documentation/devicetree/bindings/serial/cdns,uart.yaml
index 876b8cf1cafb..0c118d5336cc 100644
--- a/Documentation/devicetree/bindings/serial/cdns,uart.yaml
+++ b/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:
diff --git a/Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml b/Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml
index 2f4390e8d4e8..6f65e9a81c29 100644
--- a/Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml
+++ b/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:
-- 
2.34.1