The XHCI device index and scratchpad buffers are mapped into high memory,
in order to conserve low memory. They need to be accessible in the virtual
address space to allow us to initialise them. After initialisation. only
the XHCI accesses them.
We also only need to access the data structures in the device private
workspace during initialisation, but keeping separate physical and virtual
addresses for these structures makes the code considerably more complex,
so for now, move these to low memory.
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