sparc32: fix sparse warning in io-unit.c

Fix following warning:
io-unit.c:56:13: warning: incorrect type in assignment (different address spaces)

The page table for the io unit resides in __iomem.
Fix up all users of the io unit page table.
Introduce sbus helers for all read/write operations.

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