Introduce a want_64bit_pci support [was: PAE support]

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi,
based on the PAE discussion there is the question of what to do for
drives like twa(4). E.g. drivers that can use a more compact 32bit
layout or support the full 64bit PCI address space.

Would it be reasonable to add a new MI variable
        bool pci_use_64bit_addressing;
that gets set by MD to true if drivers should use 64bit support?
For x86, it would be enabled if physical memory above 4GB is found.
Other architectures might have similiar rules or have a IOMMU that makes
it irrelevant. The other reason for avoiding 64bit is the increased
bus traffic after all.

Comments? Especially for which platforms have to deal with this?

Joerg