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