dt-bindings: mailbox: add STMicroelectronics STM32 IPCC binding

Add a binding for the STMicroelectronics STM32 IPCC block exposing a
mailbox mechanism between two processors.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>