isa_dmamap_create() calls to their open/close entrypoints. This worked
with some luck, but broke on i386 when _bus_dmamap_create started
to allocate bounce buffers upfront, since memory below 16M may well
not be available when the sound devices is opened for the Nth time.
To fix this, create a new simple interface, isa_drq_alloc/isa_drq_free,
wrappers around already existing bitmask macros. These are expected
to be used before an isa_dmamap_create call, and after an
isa_dmamap_destroy call, respectively. For the sb and ad1848 drivers,
they're deferred until open/close.
All isa_dmamap_create calls can now use BUS_DMA_ALLOCNOW and be done
at attach time.
Based mostly on NetBSD/prep at this time since the earlier mvme160x
boards are pretty much PReP compliant. Later boards in the range
diverge somewhat from the reference platform, hence the separate port.
Still needs *lots* of bashing into shape, but at this time it will
boot to multi-user over ethernet on an MVME1603-051.
agc