Don't assume pmap_mapdev() gets called only for memory mapped I/O

addresses (i.e. uncacheable). ACPI in particular uses pmap_mapdev()
rather excessively (number of calls) just to get a valid KVA. To
that end, have pmap_mapdev():
1.  cache the last result so that we don't waste time for multiple
    consecutive invocations with the same PA/SZ.
2.  find the memory descriptor that covers the PA and return NULL
    if none was found or when the PA is for a common DRAM address.
3.  Use either a region 6 or region 7 KVA, in accordance with the
    memory attribute.

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