[SPARC64]: Add PCI MSI support on Niagara.

This is kind of hokey, we could use the hardware provided facilities
much better.

MSIs are assosciated with MSI Queues.  MSI Queues generate interrupts
when any MSI assosciated with it is signalled.  This suggests a
two-tiered IRQ dispatch scheme:

	MSI Queue interrupt --> queue interrupt handler
		MSI dispatch --> driver interrupt handler

But we just get one-level under Linux currently.  What I'd like to do
is possibly stick the IRQ actions into a per-MSI-Queue data structure,
and dispatch them form there, but the generic IRQ layer doesn't
provide a way to do that right now.

So, the current kludge is to "ACK" the interrupt by processing the
MSI Queue data structures and ACK'ing them, then we run the actual
handler like normal.

We are wasting a lot of useful information, for example the MSI data
and address are provided with ever MSI, as well as a system tick if
available.  If we could pass this into the IRQ handler it could help
with certain things, in particular for PCI-Express error messages.

The MSI entries on sparc64 also tell you exactly which bus/device/fn
sent the MSI, which would be great for error handling when no
registered IRQ handler can service the interrupt.

We override the disable/enable IRQ chip methods in sun4v_msi, so we
have to call {mask,unmask}_msi_irq() directly from there.  This is
another ugly wart.

Signed-off-by: David S. Miller <davem@davemloft.net>

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