PCI: rcar: Add multi-MSI support

Implement the MSI .setup_irqs() method which enables allocation of several
MSIs at once.

[Sergei Shtylyov: removed unrelated/unneeded changes, fixed too long lines,
reordered the variable declarations, reworded the summary/description.]
Signed-off-by: Grigory Kletsko <grigory.kletsko@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>