hw/usb/hcd-ehci: Add ctrldssegment-default property

When 64-bit addressing is supported, the Linux EHCI driver programs the segment register to zero. See ehci_run function: https://github.com/torvalds/linux/blob/master/drivers/usb/host/ehci-hcd.c The driver comment also notes that descriptor structures allocated from the DMA pool use segment zero semantics. Descriptor memory is allocated using the DMA API. The platform driver configures a 64-bit DMA mask so memory can be allocated above 4GB. See ehci_platform_probe function: https://github.com/torvalds/linux/blob/master/drivers/usb/host/ehci-platform.c On AST2700 platforms, system DRAM is mapped above 4GB at 0x400000000. As a result, descriptor addresses constructed directly from the guest EHCI registers do not match the actual system address used by the controller when fetching queue heads (QH) and queue element transfer descriptors (qTD). Add a ctrldssegment-default property so platforms can provide a descriptor address offset when constructing descriptor addresses. This allows systems where DRAM resides above 4GB to access EHCI descriptors correctly. The default value is zero, so existing machines are not affected. Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com> Reviewed-by: Cédric Le Goater <clg@redhat.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Link: https://lore.kernel.org/qemu-devel/20260713032704.3583103-8-jamin_lin@aspeedtech.com Signed-off-by: Cédric Le Goater <clg@redhat.com>