hw/pci-host/aspeed: Add AST2700 PCIe config with dedicated H2X blocks

Introduce PCIe config (H2X) support for the AST2700 SoC.

Unlike the AST2600, the AST2700 provides three independent Root Complexes,
each with its own H2X (AHB to PCIe bridge) register block of size 0x100.
All RCs use the same MSI address (0x000000F0). The H2X block includes
two different access paths:

1. CFGI (internal bridge): used to access the host bridge itself, always
   with BDF=0. The AST2700 controller simplifies the design by exposing
   only one register (H2X_CFGI_TLP) with fields for ADDR[15:0], BEN[19:16],
   and WR[20]. This is not a full TLP descriptor as in the external case.
   For QEMU readability and code reuse, the model converts H2X_CFGI_TLP
   into a standard TLP TX descriptor with BDF forced to 0 and then calls
   the existing helpers aspeed_pcie_cfg_readwrite() and
   aspeed_pcie_cfg_translate_write().

2. CFGE (external EP access): used to access external endpoints. The
   AST2700 design provides H2X_CFGE_TLP1 and a small FIFO at H2X_CFGE_TLPN.
   For reads, TX DESC0 is stored in TLP1 and DESC1/DESC2 in TLPN FIFO
   slots. For writes, TX DESC0 is stored in TLP1, DESC1/DESC2 in TLPN
   FIFO[0..1], and TX write data in TLPN FIFO[2].

The implementation extends AspeedPCIECfgState with a small FIFO and index,
wires up new register definitions for AST2700, and adds a specific ops
table and class (TYPE_ASPEED_2700_PCIE_CFG). The reset handler clears the
FIFO state. Interrupt and MSI status registers are also supported.

This provides enough modeling for firmware and drivers to use any of the
three PCIe RCs on AST2700 with their own dedicated H2X config window,
while reusing existing TLP decode helpers in QEMU.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250919093017.338309-11-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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