PCI: rcar: Select PCI_MSI_IRQ_DOMAIN

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>