hw/arm/virt: Basic CXL enablement on pci_expander_bridge instances pxb-cxl

hw/arm/virt: Basic CXL enablement on pci_expander_bridge instances pxb-cxl

Code based on i386/pc enablement.
The memory layout places space for 16 host bridge register regions after
the GIC_REDIST2 in the extended memmap. This is a hole in the current
map so adding them here has no impact on placement of other memory regions
(tested with enough CPUs for GIC_REDIST2 to be in use.)
The high memory map is GiB aligned so the hole is there whatever the
size of memory or device_memory below this point.

The CFMWs are placed above the extended memmap. Note the existing
variable highest_gpa is the highest GPA that has been allocated at
a particular point in setting up the memory map. Whilst this caused
some confusion in review there are existing comments explaining this
so nothing is added.

The cxl_devices_state.host_mr provides a small space in which to place
the individual host bridge register regions for whatever host bridges are
allocated via -device pxb-cxl on the command line. The existing dynamic
sysbus infrastructure is not reused because pxb-cxl is a PCI device not
a sysbus one but these registers are directly in the main memory map,
not the PCI address space.

Only create the CEDT table if cxl=on set for the machine. Default to off.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Message-id: 20250703104110.992379-4-Jonathan.Cameron@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>