dt-bindings: mailbox: arm,mhu: Make secure interrupt optional

The secure interrupt is only useful to secure world, therefore for NS
users it shouldn't be required. Make it optional.

This fixes a warning on Arm Juno board:

mhu@2b1f0000: interrupts: [[0, 36, 4], [0, 35, 4]] is too short
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20220728190810.1290857-1-robh@kernel.org

Documentation/devicetree/bindings/mailbox/arm,mhu.yaml

@@ -57,6 +57,7 @@ properties:
     maxItems: 1
 
   interrupts:
+    minItems: 2
     items:
       - description: low-priority non-secure
       - description: high-priority non-secure