In r225931 I've missed the only other driver using the pointer returned

In r225931 I've missed the only other driver using the pointer returned
by rman_get_virtual(9) to access device registers sparc64 currently cares
about.
Ideally ata(4) should just be converted to access these using bus_space(9)
read/write functions instead as there's really no reason to do it the
former way. However, this part of ata-siliconimage.c should go away in
favor of siis(4) sooner or later anyway and I don't have the hardware to
actually test the SX4 bits of ata-promise.c.
Also ideally the other architectures should also properly handle the
BUS_SPACE_MAP_LINEAR flag of bus_space_map(9) so this code wouldn't need
to be #ifdef'ed.