powerpc: Added PCI MSI support using the HSTA module

The PPC476GTR SoC supports message signalled interrupts (MSI) by writing
to special addresses within the High Speed Transfer Assist (HSTA) module.

This patch adds support for PCI MSI with a new system device. The DMA
window is also updated to allow access to the entire 42-bit address range
to allow PCI devices write access to the HSTA module.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/devicetree/bindings/powerpc/4xx/hsta.txt

@@ -0,0 +1,19 @@
+
+ppc476gtr High Speed Serial Assist (HSTA) node
+==============================================
+
+The 476gtr SoC contains a high speed serial assist module attached
+between the plb4 and plb6 system buses to provide high speed data
+transfer between memory and system peripherals as well as support for
+PCI message signalled interrupts.
+
+Currently only the MSI support is used by Linux using the following
+device tree entries:
+
+Require properties:
+- compatible		: "ibm,476gtr-hsta-msi", "ibm,hsta-msi"
+- reg			: register mapping for the HSTA MSI space
+- interrupt-parent	: parent controller for mapping interrupts
+- interrupts		: ordered interrupt mapping for each MSI in the register
+			  space. The first interrupt should be associated with a
+			  register offset of 0x00, the second to 0x10, etc.