dt-bindings: mailbox: ti,secure-proxy: Add missing reg maxItems

Lists should have fixed constraint, so add missing maxItems to the "reg"
property.  Since minItems=maxItems, the minItems is implied by dtschema
so can be dropped.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: krzk

Documentation/devicetree/bindings/mailbox/ti,secure-proxy.yaml

@@ -36,7 +36,7 @@ properties:
       - const: scfg
 
   reg:
-    minItems: 3
+    maxItems: 3
 
   interrupt-names:
     minItems: 1