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

Explicitly reference the dtschema for USI children implementing specific
serial protocol (I2C, SPI, UART).  The SPI schema is not yet accepted,
so it will be provided later.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20220211164716.120880-1-krzysztof.kozlowski@canonical.com

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

@@ -17,13 +17,6 @@ description: |
   child nodes, each representing a serial sub-node device. The mode setting
   selects which particular function will be used.
 
-  Refer to next bindings documentation for information on protocol subnodes that
-  can exist under USI node:
-
-  [1] Documentation/devicetree/bindings/serial/samsung_uart.yaml
-  [2] Documentation/devicetree/bindings/i2c/i2c-exynos5.txt
-  [3] Documentation/devicetree/bindings/spi/spi-samsung.txt
-
 properties:
   $nodename:
     pattern: "^usi@[0-9a-f]+$"
@@ -71,10 +64,17 @@ properties:
       This property is optional.
 
 patternProperties:
-  # All other properties should be child nodes
-  "^(serial|spi|i2c)@[0-9a-f]+$":
+  "^i2c@[0-9a-f]+$":
+    $ref: /schemas/i2c/i2c-exynos5.yaml
+    description: Child node describing underlying I2C
+
+  "^serial@[0-9a-f]+$":
+    $ref: /schemas/serial/samsung_uart.yaml
+    description: Child node describing underlying UART/serial
+
+  "^spi@[0-9a-f]+$":
     type: object
-    description: Child node describing underlying USI serial protocol
+    description: Child node describing underlying SPI
 
 required:
   - compatible