powerpc: Add simple cache inhibited MMIO accessors

Add simple cache inhibited accessors for memory mapped I/O.
Unlike the accessors built from the DEF_MMIO_* macros, these
don't include any hardware memory barriers, callers need to
manage memory barriers on their own. These can only be called
in hypervisor real mode.

Signed-off-by: Suresh Warrier <warrier@linux.vnet.ibm.com>
[paulus@ozlabs.org - added line to comment]
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>