The R-Car PCIe driver requires the use of IRQ domains for its MSI code:
drivers/pci/host/pcie-rcar.c:635:9: error: implicit declaration of function 'irq_find_mapping' [-Werror=implicit-function-declaration]
drivers/pci/host/pcie-rcar.c:666:8: error: implicit declaration of function 'irq_create_mapping' [-Werror=implicit-function-declaration]
...
Add a Kconfig select to ensure that the feature is always enabled.
This is not consistent with what the other drivers do at the moment, but I
have another patch that changes them to do it like this one, which is more
logical.