PCI: altera: Add Altera PCIe MSI driver

Add Altera PCIe MSI driver.  This soft IP supports a configurable number of
vectors, which is a DTS parameter.

[bhelgaas: Kconfig depend on PCIE_ALTERA, typos, whitespace]
Signed-off-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh@kernel.org>