hw/mips/gt64xxx_pci: Endian-swap using PCI_HOST_BRIDGE MemoryRegionOps

hw/mips/gt64xxx_pci: Endian-swap using PCI_HOST_BRIDGE MemoryRegionOps

GT64120's PCI endianness swapping works on little-endian hosts,
but doesn't on big-endian ones. Instead of complicating how
CFGADDR/CFGDATA registers deal with endianness, use the existing
MemoryRegionOps from hw/pci/pci_host.c. Doing so also reduce the
access to internal PCI_HOST_BRIDGE fields.

Map the PCI_HOST_BRIDGE MemoryRegionOps into the corresponding
CFGADDR/CFGDATA regions in the ISD MMIO and remove the unused
code in the current ISD read/write handlers.

Update the mapping when PCI0_CMD register is accessed (in case
the endianness is changed).

This allows using the GT64120 on a big-endian host (and boot
the MIPS Malta machine in little-endian).

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230104133935.4639-6-philmd@linaro.org>