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>

Documentation/devicetree/bindings/pci/altera-pcie-msi.txt

@@ -0,0 +1,28 @@
+* Altera PCIe MSI controller
+
+Required properties:
+- compatible:	should contain "altr,msi-1.0"
+- reg:		specifies the physical base address of the controller and
+		the length of the memory mapped region.
+- reg-names:	must include the following entries:
+		"csr": CSR registers
+		"vector_slave": vectors slave port region
+- interrupt-parent:	interrupt source phandle.
+- interrupts:	specifies the interrupt source of the parent interrupt
+		controller. The format of the interrupt specifier depends on the
+		parent interrupt controller.
+- num-vectors:	number of vectors, range 1 to 32.
+- msi-controller:	indicates that this is MSI controller node
+
+
+Example
+msi0: msi@0xFF200000 {
+	compatible = "altr,msi-1.0";
+	reg = <0xFF200000 0x00000010
+		0xFF200010 0x00000080>;
+	reg-names = "csr", "vector_slave";
+	interrupt-parent = <&hps_0_arm_gic_0>;
+	interrupts = <0 42 4>;
+	msi-controller;
+	num-vectors = <32>;
+};