dt-bindings: soc: samsung: usi: refer to dtschema for SPI

After adding DT schema for Samsung SPI controller, the Samsung USI
bindings can reference it directly for proper schema validation.

Signed-off-by: Krzysztof Kozlowski <[email protected]>
Reviewed-by: Sam Protsenko <[email protected]>
Acked-by: Rob Herring <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: krzk

Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml

@@ -77,7 +77,7 @@ patternProperties:
     description: Child node describing underlying UART/serial
 
   "^spi@[0-9a-f]+$":
-    type: object
+    $ref: /schemas/spi/samsung,spi.yaml
     description: Child node describing underlying SPI
 
 required: