Use BUS_SPACE_MAP_PREFETCHABLE only if BAR and driver agree on it.

- A driver that expects prefetchable memory and knows to issue the
  needed bus_space_barrier calls can pass BUS_SPACE_MAP_PREFETCHABLE
  to indicate a desire to map the memory prefetchable if the BAR
  allows it.

  (A driver that _really wants_ BUS_SPACE_MAP_PREFETCHABLE even if
  the BAR claims _not_ to be prefetchable can use pci_mapreg_info and
  bus_space_map explicitly -- this is not different from what we have
  today.)

- For a driver that _does not_ expect prefetchable memory, the
  appearance of the prefetchable bit in the BAR shouldn't cause it to
  use BUS_SPACE_MAP_PREFETCHABLE, because the driver will not issue
  the needed bus_space_barrier calls to get sensible results.

Note: `Prefetchable' here, sometimes called `write-combining', means
reads have no side effects, and writes are idempotent, so it is safe
to issue reads out of order and safe to combine writes.

Mappings with BUS_SPACE_MAP_PREFETCHABLE are often more weakly
ordered than normal memory -- e.g., on x86, in WC-type memory
regions, loads can be reordered with loads, stores can be reordered
with stores, which is not possible with any other type of memory
regions.

Discussed on tech-kern a while ago:

https://mail-index.NetBSD.org/tech-kern/2017/03/22/msg021678.html

This is option A, which received the most support.  This should help
unconfuse drivers that do not expect prefetchable mappings, like
Yamaguchi-san tripped over recently:

https://mail-index.NetBSD.org/tech-kern/2019/12/02/msg025785.html

(riastradh)