[POWERPC] Fix sparse warning in xmon Cell code

My patch to add spu helpers to xmon (a898497088f46252e6750405504064e2dce53117)
introduced a few sparse warnings, because I was dereferencing an __iomem
pointer.

I think the best way to handle it is to actually use the appropriate in_beXX
functions. Need to rejigger the DUMP macro a little to accomodate that.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>