hw/pci-host/aspeed: Add AST2600 PCIe Root Device support

hw/pci-host/aspeed: Add AST2600 PCIe Root Device support

Introduce a PCIe Root Device for AST2600 platform.

The AST2600 root complex exposes a PCIe root device at bus 80, devfn 0.
This root device is implemented as a child of the PCIe RC and modeled
as a host bridge PCI function (class_id = PCI_CLASS_BRIDGE_HOST).

Key changes:
- Add a new device type "aspeed.pcie-root-device".
- Instantiate the root device as part of AspeedPCIERcState.
- Initialize it during RC realize() and attach it to the root bus.
- Mark the root device as non-user-creatable.
- Add RC boolean property "has-rd" to control whether the Root Device is
  created (platforms can enable/disable it as needed).

Note: Only AST2600 implements this PCIe root device. AST2700 does not
provide one.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250919093017.338309-5-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>