hw/pci-host/aspeed: Add MSI support and per-RC IOMMU address space

Add MSI support to the ASPEED PCIe RC/Config model and introduce a per-RC
"IOMMU root" address space to correctly route MSI writes.

On AST2700 all RCs use the same MSI address, and the MSI target is PCI
system memory (not normal DRAM). If the MSI window were mapped into real
system RAM, an endpoint's write could be observed by other RCs and
spuriously trigger their interrupts. To avoid this, each RC now owns an
isolated IOMMU root AddressSpace that contains a small MSI window and a
DRAM alias region for normal DMA.

The MSI window captures writes and asserts the RC IRQ. MSI status bits
are tracked in new H2X RC_H registers (R_H2X_RC_H_MSI_EN{0,1} and
R_H2X_RC_H_MSI_STS{0,1}). Clearing all status bits drops the IRQ. The
default MSI address is set to 0x1e77005c and can be overridden via the
msi-addr property.

This keeps MSI traffic contained within each RC while preserving normal
DMA to system DRAM. It enables correct MSI/MSI-X interrupt delivery when
multiple RCs use the same MSI target address.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250919093017.338309-7-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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