In anticipation of the removal of the msi_controller structure, convert
the Rcar host controller driver to MSI domains.
We end-up with the usual two domain structure, the top one being a
generic PCI/MSI domain, the bottom one being Rcar-specific and handling
the actual HW interrupt allocation.
Also take the opportunity to get rid of the cargo-culted memory allocation
for the MSI capture address. *ANY* sufficiently aligned address should
be good enough, so use the physical address of the msi structure instead.