mailbox: stm32-ipcc: add COMPILE_TEST dependency

This allows compiling the driver on architectures where the hardware is not
available. Most other mailbox drivers support this as well.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>