The DesignWare PCIe driver requires MSI support, so we get a warning for
the artpec6 glue driver if that is not enabled:

  warning: (PCIE_ARTPEC6) selects PCIE_DW which has unmet direct dependencies (PCI && PCI_MSI_IRQ_DOMAIN)

Add the same dependency that all other such drivers have.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Niklas Cassel <niklas.cassel@axis.com>