dt-bindings: samsung: exynos-usi: add missing constraints

Properties with variable number of items per each device are expected to
have widest constraints in top-level "properties:" block and further
customized (narrowed) in "if:then:". Add missing top-level constraints
for reg, clocks and clock-names.

Reviewed-by: Sam Protsenko <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Krzysztof Kozlowski <[e-mail]>

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

···
       - enum:
           - samsung,exynos850-usi
 
-  reg: true
+  reg:
+    maxItems: 1
 
-  clocks: true
+  clocks:
+    maxItems: 2
 
-  clock-names: true
+  clock-names:
+    items:
+      - const: pclk
+      - const: ipclk
 
   ranges: true
 
···
         - description: Operating clock for UART/SPI/I2C protocol
 
     clock-names:
-      items:
-        - const: pclk
-        - const: ipclk
+      maxItems: 2
 
   required:
     - reg