sparc32: fix sparse warning in auxio_32.c

Fix following warning:
auxio_32.c:133:33: warning: cast removes address space of expression

To fix this auxio_power_register had to be defined as u8 _iomem.
Use proper sbus operations on the pointer.

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