1. Generic (compatible with ARM6)
1. XScale (can be used as generic, but also has certainly nifty extensions).
Define abstract PTE bit defintions for each MMU class. If only one MMU
class is configured into the kernel (based on CPU_* options), then we
get the constants for that MMU class. Otherwise we indirect through
varaibles set up via set_cpufuncs().
XXX The XScale bits are currently the same as the generic bits. Baby steps.
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