PCI: designware: Add irq_create_mapping()

Without irq_create_mapping(), the correct IRQ number cannot be
provided.  In this case, it makes problems such as NULL dereference.
Thus, irq_create_mapping() should be added for MSI.

Suggested-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>