pci_user.c: Call bus_translate_resource before mmaping bars

[kd]

Summary

On some armv8 machines there is no 1:1 mapping between cpu and pci bus BAR base addresses.
When a BAR is allocated in kernel using bus_alloc_resource_any this translation is handled in ofw_pci_activate_resource.
Do the same in pci_user.c by calling bus_translate_resource devmethod. This fixes mmaping BARs to userspace on armada7k and possibly many other platforms.