In ia64_mca_init(), don't limit the allocation of the info block to
fall within the first 256MB of memory. The origin/reason for that
limitation is not known, but it's not believed to be required for
proper initialization. What is known is that the Altix 350 does not
have physical memory at that address (by virtue of the address space
bits).

Keep the boundary at 256MB so that the info block will be covered
by a single direct-mapped translation.

While here, change the flags to M_NOWAIT to eliminate confusion. It
does not change the behaviour of contigmalloc(). What is does is
makes the flags argument explicitly say what the actual behaviour
is.
In pci_cfgregread() and pci_cfgregwrite(), multiplex the domain and
bus number into the bus argument. The bus number occupies the least
significant 8 bits. The PCI domain occupies the most significant 24
bits.

On the Altix 350, the PCI domain is a required parameter, but
changing the prototype of the pci_cfgreg*() functions to include a
separate domain argument has wide-spread consequences across the
supported architectures. We'd be changing a known interface.

Multiplexing is an acceptable kluge to give us what we need with
manageable impact. Note that the PCI bus number fits in 8 bits,
so the multiplexing of the domain is a backward compatible change.