mailbox: mailbox-test: fix null pointer if no mmio

Fix null pointer issue if resource_size is called with no ioresource.

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