sparc32: fix sparse warning in iommu.c

Fix following warning:
iommu.c:69:21: warning: incorrect type in assignment (different address spaces)

iommu_struct.regs is __iomem - fix up all users.
Introduce sbus operations for all read/write operations.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>