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>